The first thing that comes to mind is this:
<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"/>
You can position the images on top of each other with CSS, then set the opacity accordingly.
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.