Masks for css
<svg width="200" height="200">
<defs><mask id="canTopMask">
<image width="200" height="200" xlink:href="img_4.png"></image>
</mask></defs>
<image mask="url(#canTopMask)" width="200" height="200" xlink:href="img_1.jpg"></image>
</svg>
http://peterhrynkow.com/how-to-compress-a-png-like-a-jpeg/