While we're talking pictures, is there any real need for the 3.5 cm of yellow either side of the actual message boxes? If that could be reduced we could post larger pictures without having to scroll the screen.
That's part of the forum's actual page design.
I prefer to have the space at the sides than to see everything stretched out across the page. I find it makes it easier for navigation and general reading when things are grouped a little more tightly.
I think 650 pixels wide is the maximum you can go before the scroll bar appears. I only occasionally need to go larger than this. Usually I find it's ample amount of space.
It's sometimes hard to find the right settings to make the forum user-friendly to the maximum amount of people (both registered and guests), but I do try to keep everyone's best interests in mind.