The first thing that comes to mind is this:
You can position the images on top of each other with CSS, then set the opacity accordingly.
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<img id="imga" src="IMG_1.jpg"/>
<img id="imgb" src="IMG_2.jpg"/>
Is this the sort of thing that you had in mind?
If it is and you need a hand with the JS, just let me know.