Blur-use tool in Gimp - so go ahead
If you do not get the tool box in Gimp is displayed, click "window" and "Toolbox" or use the key combination [Ctrl]+[B].
- Select the tools icon from the box with the pointing Hand by the name of "smearing".
- You go over the edge in your image and click on it. To blur the edge until you are satisfied with the result.
- In the tool box you can make by the way a number of settings for the User. So you can change the opacity.

Gimp: edges blur
Read here are five of the best beginner tips for Gimp.
