1. Open your image.
  2. Select your image then press Ctrl+C so your image is copied to the clipboard.
  3. Make a new file by pressing Ctrl+N. Set the value of Width and Height the same as your image (It’s already been set by default). Don’t forget to set the canvas color to transparent. In this examples I use the values Width:375 Height:250.
  4. Select the Rectangle tool. Draw a rectangle over your new canvas. Make the rectangle size the same as the canvas size. In this examples I set the values W:375 H:250 X:0 Y:0 In the Property inspector set the roundness to 15 (or whatevever you’d like).
  5. Choose Edit|Paste Inside. Notice that your image is now inside the rectangle. Also note the little cloverleaf symbol. You can press your mouse button and drag to re-position the image.

It’s very easy, isn’t it?