I know I've seen this somewhere before but does anyone know how to stop the server from serving up cached pages?

This is particularly a problem with images. It sometimes takes several hours before the updated image appears, which can be a bit of a pain in the backside when you're developing a new site. The only way round it I've found is to rename the iamge, but that's not ideal.

I think it's possible to use a PHP header command to stop the server caching the pages but I'm not sure.