To create a deck you need at least 53 images: one for the back side and one for each of the cards (you're not forced to use a different image for every card).

To associate an image with a card, simply drag and drop the image file on the preview pane on the right. If the image doesn't have the correct aspect ratio or you just want to cut a piece of it for the card, you can crop it: click and drag to select the area you wish to use.

Once all the cards have been assigned, you can generate the final .deck file using the Deck menu.

You can also save the deck project in case you want to modify it/finish the work later.

If there's anything else you need to know, you can use the contact form to submit your questions.