Hi, you may either use JavaScript or JQuery. I have shown using JQuery. Place JQuery(.JS) file in the page when you have to use it. If you don't have .JS file than download from below code.
http://jquery.com/download/[
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Place all below code within script tag.
First take the path of the image, such as below.
var Imagesrc = $('#Image1').attr("src");
Now use that variable to assign the source of the another image, as shown below.
$('#Image1').mouseover(function(){
$('#fullImage').attr("src",Imagesrc);
});
Hope this will help you.