GNU Image Manipulation Program (GIMP) is a Free and Open Source image editor available for GNU/Linux, Mac, Windows and more operating systems. It supports effects, layers, gradients, patterns, text and many more. The toolbox has all the basic elements you need to work with photos. Furthermore, the functionality of this program can be extended by using one of the hundreds of plugins available. It can be used for producing icons, graphical design elements, art for user interface components and mockups and even more. GIMP emulates Photoshop and is totally free. You just have to get used to some changes in the way you work to start retouching photos or drawing.
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Download this free image editor from its website here.