Making round-rect selections in Gimp
First, let's create a new image
Select File->New... and select the default width and height (256x256).
Create the new image pressing Ok.
Now let's make a rect selection: Use the rectangle selection tool and drag a rectangle selection on your image
Now select Select->Shrink and shrink the selection by 20 pixels.
In the next step we do the opposite: select Select->Grow and grow the selection by 20 pixels.
Finished! Now you have a nice round rect selection!
Some of you may ask why do we need to shrink and then grow the selection, when we could have grown a smaller selection.
In this example, both methods could do just fine, but the method I used is better when you need a round rect selection of exactly 100x80 pixels(for example), as jou only
need to make sure your first selection is 100x80px.