postbackurl:
<asp:imagebutton id="CrossPostImageButton"
imageUrl="Images\CrossPost
Button.jpg
"
alternatetext="Post value to another page"
postbackurl="SomePage.aspx
"
runat="Server">
</asp:imagebutton>
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.imagebutton.postbackurl.aspx