1. Double-click the picture background layer to make it a layer that can be moved or deleted;

2. Use the rounded rectangle tool to draw a rounded square, and you can control the size of the rounded corner by adjusting the "radius";

3. Press CTRL+ENTER to convert the path into a selection;

4. Deselect the selection area, and then press DELETE to delete;

5. Save as a picture in WEB format or PNG format (leave blank).

That's basically it. Try it, hehe.