The following is a simple and straightforward Lunacore technique to add round corners to a photograph:

- Open picture
- Double click on the background layer
- Select the Rounded Rectangle Tool (U) (Make sure that Paths is selected, see attachment)
- Adjust radius
- Draw the rounded rectangle
- Select in the menu: Layer/Add Vector Mask/Current Path
- Create a new layer beneath this layer that will serve as your background.

Round Corners