If you found that your images are getting rendered at cleint little slower, you need to consider use of httpHandler to handle the images.
so, before image getting rendered at client side you can compress it.
Also, makesure that in IIS Keep-Alive is checked or set to On. Keep-Alive ensures that client's all requests are served via single connection to serer.
requests can be anyting like page is requesting image from server or CSS from server or Java scrits from server. There would be 90% boost in loading stuff at cleint side.
If Keep-Alive is to Off, client side browser opens new connection to the server for all requests like get image, css and javascript.
Making new connection to the server is very costly. You can see this performance gain by your own.

Try to compress the image using Jpeg algorithm before sending it to client.