Steps for making a transparent GIF layer in GIMP.
- Open your GIF image file.
- Open the Channels tab. You should see a single Indexed channel.
- Add Alpha Channel (Layer / Transparency / Add Alpha Channel). You should see a new Alpha channel added in the Channels tab.
- Make a selection by selecting a colour (Select / By Color then click on the area with the required colour).
- Optionally, enlarge the selection (Select / Grow, then enter the number of pixels in the Grow Selection dialog). See note below.
- Clear the selection (Edit / Clear).
- Save your GIF image file.
When making a transparent GIF with GIMP, sometimes the borders of the image can be a little ragged, especially next to a curve with antialiasing. One solution is to "grow" the selection by one pixel (Select / Grow) before clearing all the pixels for the transparent layer.