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Adding transparency with GIMP

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 -> (appropriate tool) (shift-click to add more regions/colors)
4. Edit -> Clear
5. File -> Save As … -> select GIF format
    -> export: select either Indexed or Greyscale -> save as GIF: check ‘Interlace’ option

and you get a great transparent image!

[source1, source2]

Adding scripts in GIMP

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: C:\Documents and settings\username\.gimp-2.x\scripts
global use: C:\Program Files\GIMP-2.0\share\gimp\2.x\scripts ]

and then you go to : Filter > Script-Fu > Refresh scripts
to update the script list. The scripts will be obvious at the Script-fu tab.

A good example of how covenient some scripts can be is the following Fake HDR range effect.