[EDIT]
Set the property
DefaultButton="Button2"
for panel1 so that Button2 fires.
May not be the better solution but Disabling the Enter key works but depends on your requirement. see
here[
^]
You can use Link button instead of the Image button
<asp:LinkButton ID="lnkButton1" runat="server">
<img src="imageurl" />
</asp:LinkButton>
[EDIT]