The Adblock Plus add-on is perfect for this and helps with bandwidth by blocking a whole lot of crud as well.
With the extension installed:
Right-click on the offending image and
Select
Adblock Plus: Block image...
.
(Click for a larger image)
Select your desired rule, usually choosing the prefilled
Custom
rule works best for single images.Click Add filter.
Voilà, that image is now automatically blocked from now on!
(Click for a larger image)