My MVC app creates images at runtime that I do NOT need to save on the disk. 

What's the best way to send those to the requesting browser?  

Note that the images will never be the same, so there is no reason to save them on disk first.

The server will draw the random image and will return the image to the calling client. I'm trying to understand what the best format for this type of operation is (bitmap, image ...) so that the streaming back to the server is as smooth and as fast as possible.