Set maximum size for displayed images

This sounds like a super lame question, I know, but I have several people posting who link to very large image files. Is there a way I can force these to always display with a maximum width (of say 500 px)? I’ve searched the forums and I see lots of ways to do it as the files are being uploaded, but the majority of my posters are using images hosted elsewhere, and not locally. Aside from getting more savvy posters (or editing all the posts myself) is there an easy way to do this?