Kae Verens has a recent blog post that looks at a method for helping to protect users from themselves when working with images - a method for handling ones incorrectly sized.
Every now and then, I get a call from a client who is puzzled why their site is running slow. I would look at their page and see an innocuous image inserted into a paragraph. When I examine the image, though, I see that the client has artificially resized the image using HTML. One recent example showed on-screen as a 300px-wide image. When I examined it, it was actually 3000px wide (approx).