iMac cannot open JPEG files burned to CD with Windows PC

Did you use ISO9660 (or Joliet) as your format for burning the disc? Otherwise the mac can't read it. It's not clear from your post as to whether the problem is the disc itself or the files on it. Does the mac read the disc, but just not open the files?

If it's just the files, how many different programs have you tried? Sometimes just replacing the existing extension with .jpg (even if that's what it seems to say now) can fix things.

Have you copied the files to your mac desktop and tried opening them from there? Do you get an error message? If so, what?

If the mac can't even read the disc and you don't still have access to the originals, you might try burning a new disc on a machine using XP. Just be sure to use the XP utility or choose ISO9660/Joliet in whatever program you use.

EDIT: I would leave iphoto out of the equation till you are sure you have files the mac can see.

Okay, then the problem is with the files themselves. What happens when you try to open the files? Any special error message?

You might try downloading Graphic Converter. Sometimes it can read files when other programs can't. If that doesn't work, your best bet would be to try a irfanview on a windows computer and save the files there and reburn them.

Well, there are a couple of things you could do to troubleshoot this. First, since you can read the files in Windows (can you read them on a different windows computer? If not, possibly you burned shortcuts instead of the files themselves.), try reburning one or two on a new disc and see if that helps, or if you have access to a flash drive, see if copying one of them to that drive makes them readable.

Also, did you try copying the files to your mac desktop before opening them?

Hmm, there must be something about your burning software that OS X doesn't like. Since you have access this way, I'd try copying the files over to your mac via that machine and reburning in OS X. (Do a test first to see if this will work.)