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The GIF trick is simple; if you have a 200x200 image named pic200x.jpg and a 200x200 transparent GIF named trans200x.gif, this is what to do:
<img src="trans200x.gif" style="background:pic200x.jpg" height="200px" width="200px" />
An attempt to click on the image to print or save will only capture the transparent GIF.