On a forum I go on (dunno who it's powered by, maybe yaBB or vBulletin), they have a "max picture size" set to 800x600. I have set this on mine.
However, I somtimes get Horizontal scrolling when at work (because I don't have windows maximised)
anyway, long story short, the said forum i was on about also have this feature where when you post a pic, it puts it in the post as a thumbnail at about 400x300, you then click this to see the 800x600 image. This then prevents horizontal scroll in windows not maximised