We are going to see here how to obtain 2 results involving rounded corners rectangle with GIMP:
Draw a rounded corners rectangle shape
1. Make a simple selection using the « Rectangle selection » tool:

2. Check the « rounded corner » option:

3. Go into Edition menu, and choose « Stroke selection »:

4. Choose the style and apply it:

Result:

Make rounded corners to an existing picture
1. Create a new transparent picture and open your target picture as a layer in it. Then select « Rounded rectangle… » in the Selection menu:

2. Define the radius of the rounded corner:

3. After validating we can see that our selection has the desired aspect:

4. Invert the selection pressing « Ctrl + I »
5. Press « Del » key, you’re done!
