From MozillaZine Knowledge Base

Block information

I have unblocked remote images with the button 'Show Images' for some mail. How to revoke this decision, so that the remote images for this mail are blocked again?

According to this forum thread, the address book may not be the only place where the block/unblock parameter for a specific address is kept. So, where else? --Rsx11m 13:14, 13 March 2008 (UTC)

Reblock

You can mark the email as junk. That should automatically eliminate all pictures, but it might move the email to your spam folder. Superm401 07:03, 6 Feb 2005 (PST)