Be careful with those, however. If you run a forum with a lot of traffic and disable caching you run the risk of shooting your bandwidth through the roof. Caching exists for a reason.
If you MUST control users' cache, do it in a way that you ONLY affect images that have been modified since last cache.
"Give me the place to stand, and I shall move the earth" - Archimedes
"The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts." - Bertrand Russell