ImageMask makes it possible to get Flash like image compression with alpha/transparency and small file sizes.
By adding a "data-mask" attribute to an img tag with a reference to an image that contains the mask will mask the src image.
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Then run the plugin:
$(document).ready(function ()
{
$(".container").imageMask();
});
and it will find all images with a data-mask attribute in the image container.
It combines the two images using canvas and replaces the original image with the generated one.
It requires a bit more work because one has to save two images. It also helps to have a solid background on the original image for better compression. There may be circumstances where a normal png with transparency is preffered, but with larger photos it does have a considerable effect.