May one Daven in front of a picture?
From the letter of the law, there is no prohibition to Daven opposite a picture or painting. Nevertheless, it is improper to Daven in front of a picture, in order so one does not come to stare at it and become distracted from his prayer. [This applies to all pictures, paintings, and designs. Accordingly, one who is Davening at home is to choose an area that does not contain pictures on its walls. Likewise, one who is Davening in a Sukkah is not to face any pictures that are on the walls. Likewise, one who is Davening in the hallway of a Shul is to make sure not to face the board of announcements. Likewise, for this reason one is not to Daven facing a book case with book covers that draw his interest. All the above is in reference to Shemoneh Esrei, however, prior and post Shemoneh Esrei, it is less severe to Daven opposite a picture. Nonetheless, from some Poskim it is evident that one is to initially beware of this matter throughout the entire Davening. The above restriction mainly refers to pictures that are new to the person, however, pictures which a person is used to constantly seeing are less severe if one Daven’s opposite them. Nonetheless, even so one is to initially avoid doing so.]
What to do Bedieved or time of need: If it occurred that one is Davening [Shemoneh Esrei] opposite a picture, then he is to close his eyes [during the prayer, in order so he does not look at it]. [Alternatively, one is to look down at his Siddur and avoid looking at the picture. Accordingly, in a time of need that there is difficulty finding another area to Daven, one may pray opposite a picture and make sure to close his eyes or look inside the Siddur.]
One is not to Daven facing a picture or painting, especially by Shemoneh Esrei, unless the picture is above the height of his head and cannot be seen when looking down, or there is no other area to face by Davening, in which case he may Daven with closed eyes or with his eyes in his Siddur.
Picture of a person:
One is certainly not to Daven opposite the picture of a person, as aside for the general refraining from Davening in front of pictures due to reasons of distraction, as explained above, Davening in front of pictures of a human can be viewed as if one is praying and bowing towards them. Nonetheless, from the letter of the law, if there is no other place to face during Davening, one may Daven facing the picture if he closes his eyes, or if it is above his head.
Word of advice:
In the event that there is no other area available to Daven, one can simply take down the picture for the duration of Davening and then replace it.
Which is better, to Daven on my porch in view of the outside, or to Daven in my home opposite a picture?
Seemingly in such a case it is better to Daven opposite a picture than to Daven outside, opposite the world which is a live video, and can cause one much greater distraction.
May one Daven opposite the picture of a Tzaddik, or other picture, if it helps him concentrate on his prayer and brings him to fear of Heaven?
From the Setimas Haposkim, it is understood that doing so is improper just as is the rule by any other picture or painting. However, one may certainly stop in middle of Davening to look at the picture in order to arouse his fear of heaven, and then continue Davening after the picture is put away.
 Admur 90:22; Michaber 90:23; Teshuvah Rambam 215 [Blau Edition; 20 in Fleiman edition] regarding both pictures in Shul and in house; Abudarham p. 91; Kaf Hachaim 90:137; Piskeiy Teshuvos 90:28
 Admur ibid regarding Chatzitza “Although that from the letter of the law there is no prohibition of Chatzitza” and so is implied from the wording of the Michaber and Admur ibid that it is simply improper to do so. However, see Rama ibid who writes “And therefore it is forbidden to draw pictures in Siddurim” and M”A 90:37 who writes “And it appears to me that also the walls of the Shul are prohibited from being painted with pictures” from which it is implied that the matter is an actual prohibition. Likewise, see all Poskim in next footnote who imply it is forbidden; However, see Admur ibid who abstained from using this wording of the Rama and M”A and rather wrote consistent with his previous wording that it is merely improper to make pictures in a Shul and in a Siddur.
 Alternative reason-Appears like one is bowing to the picture: Alternatively, the reason is so it does not appear like one is bowing down to the item in the picture. [See Taz Y.D. 141:6 in name of Hagahos Ashri in name of Rabbeinu Elyakim that one may not make pictures of animals in a Shul as it appears as if he is bowing to them; Avkas Rochel 63 that the words of Rav Elyakim is Ikkur [unlike his ruling in Beis Yosef]; Maharit 2:35, brought in Beis Lechem Yehuda 141; Radak; Maharam Padva 65; Radbaz 4:1178; Birkeiy Yosef 141; See Beis Lechem Yehuda 141:11 who writes that perhaps this only applies by the Mizrach of the Shul which people face and bow towards. The same would apply to any wall that people bow towards. Darkei Teshuvah 141:48 in name of Maharash Heller of Tzefas; Beis Shlomo 1:128; Dvar Moshe 1:8; Beis David Hasefaradi 5; Neta Shoreik 46; Aruch Hashulchan 90:28; Piskeiy Teshuvos 90:29; Omitted from Admur ibid]
Negation of above reason: Some Poskim negate the above worry that it appears like one is bowing to the picture, and so is the implied opinion of Admur. [Implication of Admur ibid and M”A 90:37] The Beis Yosef 141 negates this opinion of Rav Elyakim, however in his Shut Avkas Rochel he later rules like him. [Birkeiy Yosef 141] See Machaneh Chaim Y.D. 2:29; Igros Moshe Y.D. 2:55
 Beis Oveid 32; Kaf Hachaim ibid
 Piskeiy Teshuvos 90:28
 See Admur 90:20 regarding a Chatzitza who explicitly writes that it applies to Shemoneh Esrei; See P”M 90 A”A 37 who explains that the M”A 90:37 permits making pictures on the wall of a Shul if it is above the heads of the worshipers, as “one is required to look below.” Now, this requirement of looking below is only applicable during Shemon Esrei, hence implying that prior to or post Shemoneh Esrei, there is no restriction against Davening facing pictures. Seemingly, the reason for this is because the requirement for concentration during these sections is not invalidating. See also Piskeiy Teshuvos 90:28
 See Machatzis Hashekel on M”A 90:37 who explains that “also during the rest of Davening it is not common to look up” thus implying that this restriction applies also to the rest of Davening; Kaf Hachaim ibid
 See Pischeiy Teshuvah Y.D. 141:6 in name of Kneses Yechezkal O.C. 13 that for this reason one may Daven opposite the Paroches ; Piskeiy Teshuvos 90:28; See Igros Kodesh 10:136 who brings this reason to explain the accustomed allowance in making drawings and pictures on the Amud of the Chazan and negates it as a weak explanation being that the Chazanim switch daily and new people visit the Shul. This, however, would imply that if a person does have constant exposure to the picture then there is no longer any real worry of Davening opposite it.
 As the Setimas Haposkim imply that one may Davening opposite pictures even if he is used to them. Vetzaruch Iyun as why a Paroches should be different, although perhaps since it is done for the sake of beautifying the Shul therefore there is room to be lenient, although one should not be lenient in general regarding pictures even if the same reason applies
 Admur ibid; Michaber ibid; Rambam ibid
 Admur 90:20 and M”B 90:63 regarding if one is Davening in front of a Chatzitza; Piskeiy Teshuvos 90:28 that the same applies here [Vetzaruch Iyun as to why here it was omitted[
 Admur 90:20 and M”B 90:63 regarding if one is Davening in front of a Chatzitza
 Admur ibid; M”A 90:37; Elya Raba 90:27; Halacha Berurah 90:2; Shalmei Tzibur p. 109; Beis Oveid 32; Kaf Hachaim ibid
 The reason: As one is required to look down during Davening [Shemoneh Esrei] and hence won’t come to see it. [P”M ibid] Furthermore, even during the rest of Davening, it is not common to look up. [Machatzis Hashekel ibid]
 See Taz Y.D. 141:6 in name of Hagahos Ashri in name of Rabbeinu Elyakim that one may not make pictures of animals in a Shul as it appears as if he is bowing to them; Aruch Hashulchan 90:28; Divrei Malkiel 6:2; Tzitz Eliezer 19:8; Or Letziyon 2:87-11; Orchos Rabbeinu 1:57; Piskeiy Teshuvos 90:29; See 2nd opinion in Admur 90:21 and Michaber 90:22 that one should not Daven behind a person [because it appears like one is bowing to him] and that it is proper to suspect for their opinion; So rule regarding a mirror: M”B 90:71; Poskim in Kaf Hachaim 90:138 and Piskeiy Teshuvos 90:30
 See Admur 90:21 and Michaber 90:22 that from the letter of the law there is no prohibition against Davening behind a person. M”B 90:69 that when there is no other choice, one may even initially Daven opposite a person; To note, that Admur never makes mention of any reason of “appearing like one is bowing” in any of the restrictions he mentions regarding Davening behind a person or ones Rebbe