Drag and drop. See the screen shot as you might get this message. Hope this helps if not please let us know.
P.S. you need to have an application open such as photoshop, Illustrator, etc. and drag to the canvas of the application. The image should be found in Libraries and under "my Library."
(I normally do not use stock images so thus the addition to my orig. response- sorry for omitting this key element.)