Use CSS transform to flip the image. Then, use CSS mask to create an
alpha mask of the image (http://www.html5rocks.com/en/tutorials/masking/adobe/#toc-alpha-mask).
Unfortunately, CSS alpha masks do not seem to be widely supported http://caniuse.com/#search=mask. The alternative, if you
have a solid color background, is to place the iMac image in element
(likely as a background) and place a PNG-24 gradient image on top that
fades to the background color.