PNGpong... PNG is a wonderful file format that suffers from an unfortunate gamma problem. PNGpong alleviates this issue by stripping the color "correction" information out of the file using pngcrush. PNGpong then attempts to further compress the PNG file by running it through optipng (I say "attempts" because some PNGs may already be as compressed as possible).
Simply drop one or more PNG files onto PNGpong and it'll take care of the rest.
By default, PNGpong will create a new optimized file with "_PONGED" on the end of the filename. If you would like, there is a preference to overwrite the original file(s) instead.
RTWOMEY Looks great (love the little animations) and does exactly what it says. Very easy to use to.
As has been said, to drop things onto a Dashboard widget, just click and drag your PNG image, and hit the F12/Dashboard button on your keyboard. Then drop it on the widget and you're set. (Version 1.0)
ALJUK It's a widget, so how can you drop a file onto it? As soon as you touch a file on the desktop or in a folder all the widgets vanish. (Version 1.0)