Take a look at Psyki's TP ROM thread:
http://forum.ppcgeeks.com/showthread.php?t=66993
Notice how his first post goes outside of the borders due to the large picture? Might want to put a limit to the size of picture that can be placed in a post so that the picture cannot make the post wider than the content area boundaries.
Nothing against your pic Psyki, just using it as an example.
BTW, I saw a great suggestion earlier. If you don't want to make the content area wider on all themes, would it be possible to get a widescreen theme with a wider content area?