Dissolving Border

1. Load the image to be framed.
2. Channel > New Channel, #2 will be out.
3. Select > All.
4. Select > Modify > Border. # of pixels is the thickness of the border.
5. Edit > Fill. Set mode to Dissolve and color to gray.
6. Deselect.
7. Image > Adjust > Invert, to invert the border.
8. Export with Gif89a. Set Transparency from Channel #2.
9. Voila!