You can handle that using jQuery or JavaScript code which ever suits your document requirement. I would try to be short, so I will use jQuery.
$('body').keydown(function(e) {
if(e.keyCode == 13) {
$('img').trigger('click');
}
});
This would do it, it would check for the keydown event of the body element. And if the key pressed was Enter key, then it will trigger click event on the image element.