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">