Ideally there should be no barrier between one praying the Amida and the wall, so that nothing distracts her from prayer. Permanent furniture standing against the wall, such as a closed cupboard, is not considered a barrier since it does not cause distraction, and le-khatĥila one may pray next to it (SA 90:21; MB 63:65).
Pieces of furniture which were made for praying purposes, like shtenders (lecterns), are not considered barriers. Likewise, a table on which one puts her siddur is not considered a barrier. However, when the table is of no use, it should not be allowed to divide the person praying from the wall (MB 90:66; Peninei Halakha: Prayer, ch. 3 n. 6).
If a person is standing between a woman praying and the wall, if that person is also engaged in prayer, he is not considered a barrier. However, if he is not engaged in prayer, he is indeed considered a barrier (SA 90:22; Rav Kook in Tov Ro’i on Berakhot 5b).
It is not proper to pray facing pictures, lest it be a distraction (SA 90:23). However, if the picture is above eye level it is permitted, for then there is no concern that people will be distracted by it while praying (MA 90:37; MB 71).
One may not pray facing a mirror because one who does so looks like she is bowing to her own reflection. Therefore, even if she were to close her eyes, it is still forbidden (MB 90:71). Le-khatĥila, one should not pray at night in front of a window in which her image is reflected, since looking at her reflection will likely disturb her kavana. But if there is no alternative, she should close her eyes or look in the siddur. Since the window does not reflect her image clearly like a mirror does, she does not really seem to be bowing to her reflection (see Peninei Halakha: Prayer, ch. 3 n. 7).