The GNU Image Manipulation Program

 GIMP is an advanced picture editor. You can use it to edit, enhance,
 and retouch photos and scans, create drawings, and make your own
 images. Lots of tools are available; you can sharpen and resize photos,
 and remove dust and red-eyes, for example.
 .
 It has a large collection of professional-level editing tools and
 filters, similar to the ones you might find in PhotoShop. Numerous
 fine-control settings and features like layers, paths, masks, and
 scripting give you total control over your images.
 .
 Many image file formats are supported, including JPEG, PhotoShop
 (.psd), and Paint Shop Pro (.psp) files. It can also be used to scan
 and print photos.