Client caching of image returned from url

[Pål Johansen]

Hi all
I have an jpeg image(Dynamically created) that is returned from a page as
binary stream.
Also i have set the cache policy as public, but the client refuse to cache
the image.
The page using the image does a postback everytime the user clicks on the
image imagemap and reload the image from the server every time..

I guess the client don't cache the picture because it hasnt an image
extention. ?
I have set Response.ContentType = "image/jpeg"

Anybody has an idea how to solve this?