GIMP (GNU Image Manipulation Program) is a free software raster graphics editor. It is primarily employed as an image retouching and editing tool and is cross-platform.

written tutorial - Color to transparent in GIMP
21 Feb, 2012
How to change color to transparent in gimp. For an example it's useful for removing background color from logo.
written tutorial - Remove Objects From Photos With GIMP
10 Dec, 2010
This tutorial shows how to remove objects, smudges or items from photos and images. We are using GIMP for it.