first thing you need to do, is open your image you want to make the “edges” round.

now zoom > “fit to screen.” to zoom, select the magnifying class icon, and right click on the image and select “fit to screen.”

once you do that, use the rounded “rectangular tool” and select “paths”, and set the radius to 10-30, or what ever you like.

start at the top-left corner of your image, and drag all the way to the bottom-right of your image.

once you do that, on your littler dock, select the “paths” tab. and press the rounder dotted circle. a selection should come up.

now, on the top of your menu, you should see file, edit, etc…. click the select > select inverse.

you should see the selection inverse, now, press backspace, and it should be gone! (make sure your background color is set to white).

there!!! i hope this helps. note: there are more ways to do this, im just showing you how i do it.