A short guide on using GIMP to create the avatar images.

  1. Make sure that your layer has an “alpha channel”:
    Layer → Transparency → Add alpha channel
    

    If it’s grayed out it means you already have one

  2. Create a circular selection with the “Ellipse select tool”. Use the “Tool options” dialog to set the ratio to 1:1:
    Windows → Dockable dialogs → Tool options
    
  3. Once you have the required selection, invert the selection so that everything except your circle is selected, and delete. You should have your central circle left, surrounded by a checkerboard pattern. This checkerboard is not part of the image, it just indicates the transparent parts of the image.

  4. Crop the image:
    Image → Crop to content
    
  5. Rescale the cropped image to 128x128 at 600dpi.

  6. Export the image to a format that supports transparency, like PNG