So I tried both SteamVR's media player and Big Screen back to back and I've got to say Big Screen is definitely better for mainly one reason: you can switch photos easily with arrow keys (left & right) using Windows slide-show in full screen mode (space-bar pause if you need to). Where SteamVR's media player can switch using up and down arrow then Enter key to switch but will jump out of stereo mode depending on the photo (all have "_vr" at the end of their names) not to mention the extra keystroke (Enter) to switch but it doesn't cycle from end to beginning and the jumping out of stereo mode is intolerable when compared to using the free Big Screen steam app. One issue with Big Screen is you have to open the stereo photos in windows into windows old Slide-show photo viewer (or maybe the new version) but who cares when compared with the other issues/deal breakers. Glad that Steam VR has a way to do it, it's just inferior by a mile to Big Screen. Also you get a nice living-room atmosphere with Big Screen (or pick a different setting).
Happy viewing ^_^*