When we need to clip an irregular shape from an image and use it on the web, PNG is the most common answer as it is the only image format used by all web browsers that support alpha channels.
However, PNG images come with a penalty of size (if the source is a photograph) compared to similar sized JPEGs.
polyClip.js is a script, built on jQuery, that allows us to clip any image with
imagemap coordinates using HTML5 Canvas.
For implementation on many images, "how practical would it be to use polyClip.js" is open to discussion however, for only few images, it can be a huge byte-saver.
And, it is supported in most browsers including IE7 (with Excanvas).