Can someone please tell me how to replace a standard button with a custom image button?

I am using the following code to render a button :

    <li><ext:Button ID="Button1" Text="Create Clinic" runat="server" ></ext:Button></li>
I would like to use a custom button e.g. CoolButton.png to replace the standard button.

Also how would I then create handlers for the image button ?

A simple example will be highly appreciated and thank you in advance