Well, read this article first before finding other solution http://support.microsoft.com/kb/921277
I've been trying to look around and work on the code but it's not working. Don't waste your time for simple problem unless you have plenty of them.
The easiest way is change the ImageButton control to Button control as stated in the article. Put in some css will give the same effect/result in term of display like in ImageButton.
Below is the css that i use for this purposes.
Try this http://www.pagetutor.com/button_designer/index.html if you want to customize your button css. Very useful for me.
Remember set UseSubmitBehavior for Button control to False in Properties window.
it's work for me. My problem solved.