(see previous post for fixing avatars and general explanation of what's going on)Edit #2: When I went to the Civ4 forum and looked at several threads, everyone's avatars show fine but all of the screenshots are missing. Instead of a screenshot image, it just shows a code number with either a PNG or JPG suffix. The codes are not links just "names" for the missing screenshots.
After fixing avatars in Firefox 31, I tried a Civ4 thread, and saw what you describe. I suspected the same thing, and now that the images are hosted on Cloudflare, looked at the certificate for one such image. Sure enough, it's an ESET certificate that Firefox 31 doesn't recognize. This time I captured the warning page:
Link to one such Civ4 image. Click it in the old Firefox and you should get a similar page.
I added the exception, refreshed the Civ4 thread, and the images were loading.
You may have to follow a similar process with other images hosted at third-party sites that aren't loading; it is likely to be an increasingly common problem as Let's Encrypt and ESET are increasingly popular certificate providers these days. But that should get the most common images working for you.