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!
