How to crop a rectangle with rounded corners.

[Ofnuts]

There are only rectangles... but some rectangles can have transparent pixels in the corners. If you open the image in Gimp, you can use the rectangle selection (with its Rounder corners and companion Radius option) in two ways for this:

1. Make a selection, copy, switch to the target image, paste
   
2. Make a selection, invert the selection  (Select ➤ Invert) and [delete] then drag to your target image.