You can select photo from one of your albums and place it in the empty frame. Then edit the shape and size of the photo if needed by dragging the corners. To place multiple photos, you can use the layout templates from the editor (on the left) or create your own layout adding the image(s) from the menu above your creation.
When creating your card, make sure that you edit all sides of it. The cards will have text inside them, which is added by default to show you where you can insert your own personalised text. This default text can also be deleted if you prefer to leave the space blank, allowing you to write your own text by hand if you wish.