How do I cache the images properly, I think asked this somewhere before, but it hasn't affected me until gameing site went out of beta.

It's HTTP headers or something isn't

Ok I used George's answer but frankly the performance is awful, asking the server for the image everytime (even when it doesn't download the image) creates a small delay of about 1/2 a second but because of the huge number of SE sites, the 1/2s add up.

Please, please consider adding a hash of the image to the stackauth.com/sites