A note about compression. There are two kinds lossless and lossy. Lossless leaves you with all the data that you put in. With images it doesn't do much for images. Rar and Zip are lossless.
Lossy compression is like JPEG, PNG, and others. They work by removing data and replacing it with mathematical functions. They approximate the removed data with the values the function predicts. Loose image quality and computer power but gain smaller size.
I use gimp from gimp.org it is a free photoshop program. It will run on linux, windows and Mac (I think you need X-Code on mac).
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