All dynamic pages have noCache on by default.
But you're not talking about dynamic pages, you're talking about images, which I suspect you're storing on the disk. These are static files, not dynamic ones.
If you wish to make some static files no-cache, then you need to set self.ForceNoCache, but on a selective basis. ie test the self.RequestFileName property first. In order not to clash with the static images, which are in the images folder, I recommend putting dynamic images in some other folder, and testing for that folder name in the request.
Bear in mind that when you include an image in a browse, all the browse generates is the <img> tag. The browser then fetches the image as a separate request, and hence on a separate thread, so this code goes in the WebHandler, not in the browse.
Cheers
Bruce