Chris, apologies if it's been mentioned previously (I don't really have the time to sift through the 35pp of comments), but my biggest criticism is that it's difficult to move through a series of images. I'd really like to be able to hit the left/right buttons on my keyboard, or hit an embedded button in the image to load the next one in that category. There are Next/Previous buttons, but when you hit the next image, the window resets to the top of the page, and you have to scroll down to see what the next thing is... then go find the buttons again. Likewise for the fullscreen mode.
My only other concern is that looking at images is fundamentally different from reading a thread -- the image should be big, it should be centered, and the comments, image stats, number of times it's been liked, etc are really secondary to that. Make sure you're keeping the focus where it belongs. As I mentioned above, looking for next buttons, scrolling down, etc all take away from the enjoyment of looking at nice photos.
Bottom line is that I really like what you've done so far. And I especially like the "group" aspect of the categories. I see this getting being useful for an event -- create a category for it, and everyone can post their images to the same place. However, the basic useability/ui part still needs some work.
:beer: