ImageOptim – Image Optimization Tool

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ImageOptim is a free app that compresses images to take up less disk space and load faster without sacrificing quality. It is achieved by optimizing compression parameters, removing junk metadata and unncessary color profiles.

This article at Bjango talks about PNG compression for iOS along with comparing ImageOptim with other options like Save As and Save For Web in Photoshop and the default compression system in Xcode.

If you want to find more similar tools, here is a list of 18 image compression tools from Mashable.