When input type="image" , it acts like a submit button and will cause a page to refresh. Try using this on the page itself, and you should see it causes a page refresh. If you want to perform client-only action using an image, you have two options.
1. Return false from within the onclick method to stop post back.
2. Use an img to display an image and hook an onclick event to it.
I hope this helps.
Taz.