To modify an existing image so it has transparency in Gimp, here’s a quick summary of steps from this section of the Gimp docs:
- Open the image
- Add an alpha layer: Layer > Add Transparency > Add Alpha Layer
- Show the Channels dialog: Windows > Dockable Dialogs > Channels – you now see one of the original image and the Alpha layer
- Select the original layer, then use the Eraser tool to erase the original background from the image (the part that you want to be transparent).