Could you try disabling graphics hardware acceleration? Since this feature was added to Firefox, it has gradually improved, but there still are a few glitches.

You usually need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.).

orange Firefox button or classic Tools menu > Options > Advanced

On the "General" mini-tab, uncheck the box for "Use hardware acceleration when available"

If you restart Firefox, do JPEGs render correctly?

If problems persist in multiple browsers, I wonder whether you have any connection "accelerator" or other intermediate software that could be damaging the files.

Which security software (firewall, anti-virus) do you have?

It is possible that your anti-virus software or firewall is corrupting images, so check the advanced settings for Firefox to make sure that nothing gets blocked.

Reload web page(s) and bypass the cache to refresh possibly corrupted files.

• Press and hold Shift and left-click the Reload button.
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Cmd + Shift + R" (MAC)

jscher2000 - I tried disabling the acceleration, but the problem was the same when I restarted Firefox. The connection accelerator or other intermediate software is a possibility--any sense of what sort of programs I should look for that might do that?

cor-el - I'm using McAfee. The Shift+Reload thing actually DID work. Do you know if there's a way to set bypassing the cache as a default for certain pages? Or is that a really bad idea?

Thanks, folks!

It is not a bad idea, but it just shouldn't be necessary to use such measures.
Images and other downloaded files shouldn't get corrupted if there is a good connection.
You seem to be suffering either packet loss or packet corruption that causes images not to be displayed properly.

Your system details list doesn't show that any network.http prefs are changed, so that isn't causing it.