I never touched the CSS.
The images aren’t too wide – you’re right in that they knock the sidebar down if they’re too big, so I have to be careful. And it only started messing up yesterday, and the last post with images has been there over a week.
Actually, I’ve just went on uploaded ALL the original files in the folder for the default theme and things are still messed up, so is it not actually anything to do with the theme at all?