You can do the same using the free Gimp Software.
Here are the steps:
- open up Gimp
- load your image
- go to "Ebene -> Transparenz -> Alphakanal hinzufügen"
- go to "Ebene -> Maske -> Ebenenmaske hinzufügen"
- make sure that white is fully solid (non-transparent) and click ok
Now you can draw using grey scale colors to define the parts which should be transparent.
At last step you need to save the image as .tga file