Why are you trying to use jQuery for this when simple javascript works fine?
A clumsy, but functional way is to use the .attr handle to pass variable in the <img> tag to jQuery. eg
Code:
<script type='text/javascript'>
$(document).ready(function(){
$(".img_thumb").mouseover(
function () {
var var1 = $(".img_thumb").attr('var1');
var var2 = $(".img_thumb").attr('var2');
var var3 = $(".img_thumb").attr('var3');
expandthumb(var1, var2, var3);
});
});
</script>
<img class="img_thumb" src="thumb.png" var1="1" var2="657" var3="623" width="300" height="50" />
Hope this sets you down the right path