So with the new layout the whole 'don't post images that stretch the layout' rule is kind of outdated because an image that doesn't stretch on one screen might stretch another. /:

So I'm just wondering if there is an actual size limit like for forumtext?

Yes, there is a definitive limit. A user forumtext and signature can be no more than (combined) 325px tall and 450px wide. Text counts at 10px per line.

No, I meant images in a post, not forumtext, heh.

Images must also abide by those rules, for the most part, especially with width, as that is the part most likely to stretch the forums.

Edit: Spelling error.

any image that is wider than 400 px stretches the layout on my mom's laptop (17" monitor I think) and my computer (1440x900 res 19.5" monitor).