Lately I was making a template which had a working AJAX Contact Form, at first I struggled a little for the loading image, but then I realized there are couple of websites where we can actually get(and generate) amazing loading GIF images easily according to our specifications, most of these sites also provide a wide range of images to chose from. Thus I am sharing some of those websites, hope this is helpful.

Preloaders.net

Ajaxload.info

Chimply

Web Script Lab

Load Info

AJAX Activity indicators