How to display upper half of image in asp imagebutton

Hi,
I try to replace a html image link button with asp imagebutton. Initially it should display the upper half of src image for hovering effect. While using <a hef="link1"><img src="image1"></a> is perfectly fine, using <asp:ImageButton runat="server"
ImageUrl="image1" PostBackUrl="link1" /> displays the entire image (scaled). So how to display image's upper half in asp imagebutton?
Thanks.

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