the mod created by anguz is fine enough... 
uses CSS to only display an area.. but.. is better as a mod...
that's another good option...
I thought this feature was already in even YaBB SE...?
Max width/height... makes your forum intolerably slow... ring any bells?
-[Unknown]
well, if you take a look at the forum i listed in my initial post, you'll see that is does indeed take some time to load, but that's just because most pages over there have like 10+ large images to load... The resizing itself doesn't seem to take that long...
Plus, you have to realize that an average page does not contain that much large images... not by far, so i don't really see a problem in loading times here... maybe the technique used in YSE was a bit inefficient? (although i have never seen it with my own eyes in YSE.. )
Anyway, i think most people wouldn't mind the minimal extra loading time.. for the sake of readability

ANY webdesigner knows that the worst thing a webpage can contain are horizontal scrollbars.. it's just not-done.
My conclusion: I'd REALLY appreciate it if one of the dev's would implement such a feature in SMF, as an admin option.