Yesterday I needed to supress the submit functionality of some HtmlInputImage controls (aka <input type="submit" ... /> ) and found the answer with a bit of Googling.  I'm posting it here with what is (hopefully) a better heading for the solution.   Perhaps Google will index this page and make it easier for someone else to figure out.

In your form tag, use the following:

<form id="form1" runat="server" onsubmit="return false"> 

This will prevent the standard submit behaviour and allow your onclick JavaScript to run as you expected.