The Shulhan Aruch (O.H. 90:23) says that it is not proper to pray in from of garments that have drawings on them since they can distract from one’s concentration. The Rama (ibid.) adds that one should not pray from a Siddur with any drawings, for the same reason. The Shulhan Aruch says that if one has no choice but to pray in front of a drawing, one should
Rabbi Yitzhak Ben Oualid (Vayomer Yitzhak, vol. I, Orah Haim, § 10) applies this Halacha to the Parochet (the curtain that covers the Hechal) and says that it should not contain elaborate drawings as it may detract from the concentration of the congregants. [There is a separate discussion regarding praying in front of images and three-dimensional objects from the standpoint of Avoda Zara]. Rabbi Moshe Malka (Mikve Hamayim, Yore De’ah, § 19), on the other hand, does cite opinions which permit a Parochet even with elaborate drawings, so long as there are no three-dimensional images. Some rabbis have a middle-of-the-road approach and allow a Parochet with a very basic design but nothing more. Placing pictures of rabbis in the sanctuary is inappropriate as it may also detract from one’s concentration.
The Mishna Berura (M.B., O.H. 90:71) discusses the propriety of praying in front of a mirror and says that when doing so it appears as if one is praying to an image of oneself. As such one should not pray in front of a mirror, even if one prays with one’s eyes closed. Rabbi Ben Zion Abba Shaul (Or Lezion, vol. II, ch. 7, § 11) rules that one may pray in front of a window since, unlike a mirror, one’s reflection is not so apparent. However praying in front of a picture of a rabbi, he says, is not appropriate, even if one closes one’s eyes. Similarly, if one prays at home, one should not pray in front of a family photo or the like.
Thus in designing a synagogue, one should keep in mind to minimize the artistry that is used in the sanctuary. If images need to be used, they should at least be placed high up. Rabbi Shlomo Zalman Auerbach (commentary to Rabbi Avigdor Nebenzahl’s glosses to the Mishna Berura) says that people tend to look in all directions while praying so even placing images high up may be problematic.
Summary: One should not pray in front of anything that may detract from one’s concentration, such as an elaborate image, a mirror or a picture of somebody.