A Quick Vignette, Using the Gimp

It's easy to fade out the edges of a picture using the Gimp. Here's how: (using Gimp version 2.4)

1. Decide how broad you want the fade to be
2. From the menu, Layer → Transparency → Add Alpha Channel; if this option is disabled (greyed out), then the current layer already has transparency enabled and you can skip this step
3. From the menu, Select → All, or on the keyboard, Ctrl+A
4. Select → Shrink
5. Shrink the selection by half the breadth you chose earlier, ensuring that Shrink from image border is ticked
6. Select → Feather
7. Feather the selection by the full breadth you chose earlier
8. Select → Invert or Ctrl+I
9. Edit → Clear or Delete

If you want to fade to a solid colour rather than transparency, skip the final step and instead use the Bucket Fill Tool with the Affected Area set to Fill whole selection.