Photokey can work with up to 3 layers of images.
1. Overlay (with alpha channel)
2. Greenscreen photo
To create a greeting card, you will need at least an appropriate background image to place behind your photo, though it works much better if you have an overlay to match. If you have a image editor and some knowledge of alpha channels, you can make your own overlays.
A good alternative is to have a look at the Greeting Cards overlay pack:http://fxhome.com/overlays/greetings-cards
which has ready-to-use backgrounds and overlays which can be used to create cards for a wide variety of occasions.
If you need more specific info about any of the process, let us know and we'll help you out.