Newbies, don’t upload exploits with your images!
Manuel Lemos from phpClasses.org has written an excellent post about about the risks when allowing users to upload images. It’s not something I’ve done alot but lets say you have a generated image such as this one – with a .php extension. I could upload it to your server and have it served from there – not much use to me because it would fail to update.
The next guy, however, might be uploading a malicious script that might take over your server, send spam etc.
Read his post here: PHP security exploit with GIF images