Once you have selected a rectangular or square area in Gimp, you can give it rounded corners and feathered edges.

You can change the resize settings for your selections and choose whether to expand your shape from the center, or a corner. You can choose whether you want the Height or Width to be in a fixed position, so that when you click and drag them they do not change. We also look here at how to highlight your selection to make it stand out, and how to apply guidlines to them.