|The simple answer is that you have told Mozilla not to display the images. On the preferences page "Privacy & Security", "Images", see what you have set the image acceptance option to. It could be "no images" or "only from originating server". You can also allow or block them from certain sites with "Manage Image Permissions" on the same preferences page.
If the preferences isn't the problem, try creating a new profile to just check that it is working properly. If this is OK, then it what extensions have you added to Mozilla? If it isn't OK, I'd suggest reinstalling Mozilla, as it sounds like something is broken in the install.
The only other possibility I can think of, is that you are using a cache or filter tool (such as SmartCache) that can block or change the html so that things like ad images are not displayed.