You can crop photos tighter to fill a card (square) or wider to include a full horizontal picture.
When you add a photo, you'll see the following two cropping options:
If the image is not square and un-cropped, then we take a portion of the image to use as a background instead of a thick black border. We blur that background a bit so that it is a nice graphical treatment of a border (pictured above).
The green lines are just guides on the screen, they don't print.
You can use your cursor to select any corner of the photo to resize the crop. You can also click the three options below the photos to crop or flip the photo.
If you've already added a photo to a card, click on the pencil icon in the upper right corner of the card to get to the same cropping function pictured above.
Then, select Adjust Photo: