How To: Set Image button as default button

<asp:Panel id="pnl" runat="server" DefaultButton="imagebutton1">

// Some of your controls and code //

<asp:ImageButton id="imagebutton1" runat="server" ImageUrl="images/imge.gif" >
</asp:ImageButton>


// Some of your controls and code //

</asp:Panel>


Reference: StackOverflow.com