a few things, layout-wise.
You know, I have 1680x1050 screen resolution, and it's considered rather low nowadays. So, why in the world would you limit the thumbnail div container to 900px?
http://puu.sh/24pQR
No, really: http://puu.sh/24pRU
You could at least center it.
Next, I can probably understand why you decided to generate previews, but FFS, 850px wide? WHY? Not only it is creating a huge overload on the server...yes, I saw that 'out of memory' error in upload queue and encountered it myself back in the days, image processing libs are damn greedy when it comes to huge resolutions.
...but is also mighty useless as it is now. I mean, come on, even 1024x768 wallpaper is resized. It might be a little easier on traffic, yeas, but preview generation overhead and upload queue which can hang the entire site are so much worse. And you left us without oh-so-convenient 'resize to fit the screen' option. And without the option to resize it back once we are viewing the full image.
Usability-wise the better solution would be 'resize to fit the screen' and 'resize images over <x> pixels wide' options in the user profile. Done rather easily with javascript, no overhead, old uploading process, less grumbling from the users :3
And I'll say this again, but the new pool positioning is awful. If I got your idea right and you want the image to be 'in the center' as much as possible, then move pool list below the image, above the comments. They often have long names, which are hard to read when they are that cramped, and navigation became a bad joke with those tiny « ».