Tools > Options > Content

Load images automatically = Exceptions...
highlight and use Remove Site for exceptions you inadvertently set

See also http://kb.mozillazine.org/Images_or_animations_do_not_load

You can see the permissions for the domain in the current tab in Tools > Page Info > Permissions
Some websites may load images from a different domain and in that case you need to check in Tools > Page Info > Media which images are blocked and remove the check-mark from "Block Images from..." after selecting a grayed image.