It's the stupid forum's autoresizing to forum width.
Hmmm... its force-resizing to 1332px of width on all large pics instead of doing a sensible scale-down of the full picture... Bug perhaps?
Sample post:
Link
Its mildly annoying since I now have to open it up on Phyrefile to see the actual size before I can choose wallper or not
EDIT: after some poking around, I found that the image loads up properly, but the image size detection seems broken and stuck at 1332px wide (correct AR so vertical pixels is to the correct scale) and it won't expand completely when you click it, reverting to the 1332px wide maximum. 1680x1050 screen with FF6.0 Nightly and Chrome 11 in case you're wondering.