I think it's really the small space and dark wall & tile colors that pull it together. Subj matter would not have to be same but no colored pics is a must.

Staring at it, the pic frames are about the size of tiles so that it seems like a continuation to the eye.

I think there are several design factors at work here. First is color. As Azstrawn pointed out, the subject matter of the photos is "quiet" in color through the use of black, white, and sepia tones, thereby allowing the frames to play a heightened role in the collage. The frames and enclosed photos carry out the brown/bronze, black, and gold scheme nicely with only small touches of white or cream...which echo the "bright spots" of the sink and light sconces. Secondly, the frames and/or mats are of similar size and shape with approximately the same expanse surrounding each photo...a principle of formal arrangement in keeping with the overall style of the room. Third is similarity of texture...natural wood...carved, marled, or embossed, echoing the textures of the carved vanity, wall, and floor...and with touches of gold that reference the molding that runs along the wall above all the photos. Finally, note the way the frames are placed in "line" with each other...almost, but not quite, in straight lines...enough so to give the sense of uniformity, but not in rigid straight lines which would have been uninteresting and would have required frames of exactly the same size and shape...boring. This is in harmony with the arrangement of the slate tiles on the wall, the uniformity of which is relieved by the interspersed gold diamond tiles. I realize that this answer comes months after your question, but I hope it might still prove helpful.

This is such a neat idea.. I will start buying frames at thrift stores and garage sales and bringing pics into Photoshop and age them