FADE ALL OF THE EDGES OF AN IMAGE.
Open your image in GIMP and choose the Rectangle Select Tool.
In the Tool Options Dialog, select Rounded Corners and choose a radius for the corners. For this image, I'm using a radius of 25 pixels.
Select the portion of the image you want to keep by dragging a rectangle in the graphics window.
At this point you can adjust the radius of the corners if you like.
Now go to the menu and choose Select->Invert.
Go to the menu again and choose Select->Feather. A window will pop up for you to choose the amount to feather the selection. This will control how much of the image will be faded out. For this example I'm using 50 pixels.
Now to create the fade, just hit the Delete key on your keyboard or select Edit->Clear.
Your edge fade effect is now complete. Just save your image under a new name and you're done.