Open your image with GIMP and: 1. Layer -> Transparency -> Add Alpha Channel 2. Select either by colour or selected area: a. Select -> By Colour -> click on a colour (choose additional colours by holding down Shift and clicking again) b. Tools -> Selection tools -> …
GIMP is a very powerful image processing program, equivalent to Adobe’s Photoshop, but with the biggest advantage that is totally free (and open source). Apart from its own capabilities someone can add more scripts (.scm files) in the folder: local use: ~/.gimp-2.x/scripts global use: /usr/share/gimp/2.x/scripts [or for Win local use: …