There may be a simpler way but this is what I do. The image you opened does not have a transparent background and will simply display white in the areas you cut or erase. Select the foreground image you want to save from your photo, cut and paste it into the transparent window you created with the project. You can also cut and paste the entire original image to the transparent project window to begin with so it will display the transparency underneath as you trim it out.
R Summerfield