GIMP Tutorial 2 - Vignette

1. Add an image.
2. Select Rectangle Select Tool and select area you want.
3. Click Quick Mask Button.

4. Filters -> Artistic -> Cubsim.
5. Can add more specification. Filters -> Blur -> Gaussian Blur.
6. Now click Quick  Mask Button again (off).
7. Now we want to select the border of the image. Select -> Invert.
8. Select any foreground-color and right click on the image Edit -> Fill with FG color.

9. Now Select -> None.

10. Save image as your desired format.