You’ll always get mouse co-ordinates for a submit button type=”image”
You can use a standard submit type button and just apply styles to it to change the look.
<input type="submit" id="search-submit" value=""
style="background-image: url(/images/search-button.gif); border: solid 0px #000000; width: WIDTHpx; height: HEIGHTpx;" />