There are various solutions exist for compressing and minimizing the size of images (like Smush.itPNGGauntlet, punypng, Kraken.io and more).

Here is another good one: TinyPNG that only works with PNG files, shrinks them -nearly- without loss that an eye can see and by keeping the transparency.

TinyPNG - Image Shrinker

It works by drag 'n' drop uploading the images to be minimized to the web interface and auto-generates the compressed ones.

Using TinyPNG, it is easily possible to get 50%+ smaller sized images as they are converted to 8-bit PNGs (rather than 24bit) by using smart techniques.

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