(I feel dumb because I just made a thread about this, and then I saw your reply here)
This doesn't appear to work the same way as a "backward" and "forward" button would, though, because it's not in any particular order. If you click the thumbnails in the "older" or "newer" columns, they are not the pictures immediately preceding or following the current picture in that folder; they seem to just be randomly selected from before or after it within the folder. I've had two "newer" submissions show up in the bottom right, clicked one, and then scrolled down again, and it shows there are no "newer" submissions following that, and the ones in the "older" column are completely different than the ones I saw on the previous submission and do not include the previous submission either!
Maybe I'm just encountering a glitch, or something, but the "older"/"newer" function is sort of pointless if it just randomly picks two older and two newer thumbnails for any given submission. Either that, or I'm missing something very important, and I'm just dumb.