If you're using the built-in image blocking in Firefox 1.5, go to Tools-> Options then select the Content tab. Make sure that Load Images is checked, for Originating Site Only is unchecked, then click the Exceptions button on the right to remove anything that may be blocked by mistake.
If you're using Adblock, click Tools-> Adblock, then go to Preferences and make sure you aren't blocking a site you want.
I hope this is of help.