ImageOptim optimizes images — so they take up less disk space and load faster — by finding best compression parameters and by removing unnecessary comments and color profiles. It handles PNG, JPEG and GIF animations.
https://www.pureftpd.org/project/libpuzzle
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The Puzzle library is designed to quickly find visually similar images (gif, png, jpg), even if they have been resized, recompressed, recolored or slightly modified. The library is free, lightweight yet very fast, configurable, easy to use and it has been designed with security in mind. There is a PHP extension.
Image Optimization, Part 3: Four Steps to File Size Reduction » Yahoo! User Interface Blog
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https://clarle.github.io/yui3/blog/2008/11/14/imageopt-3/
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This post is about some common tools you can use to reduce the file size of your images. The idea is to be able to just take the images your designer has created and instead of using them ?as is?, go ahead and tidy them up in short time and no effort, without even looking at them.
Browsers differ in rendering images at non-native sizes, with some providing noticeably better results than others. Internet Explorer 6 in particular looked to be the least smooth when scaling images down, but some interesting workarounds are available.
SmushIt is an online tool that optimizes images. A firefox extension is available, that allows to optimize all the images of one web page at one stretch.