Facebook cover photos need to be a particular size: 851 pixels wide by 315 pixels high.
When purchasing a Facebook cover photo from the online store, you choose which image you want, select the Facebook Cover Photo option, if it's available, and your shopping cart is updated.
Normally, you have to do nothing more when purchasing images for print or download, but with Facebook Cover Photos, you have to select which portion of the image you want as the dimensions of the original image will be different from what you need as a cover photo.
Also, all of the images on the site have a white border round the edges by default; which you will be unlikely to want in your cover photo.
First simply select the photo that you want and add to your shopping cart and proceed as normal. At the checkout you will see something like this:
If you click on the image or the red text “Centered (is it OK?)”, you'll see something like this where you get the option to select the portion of the image you want:
Simply click on the corners of the orange box and drag them to re-size the crop. You can also move the box around to select the portion of the image you want:
When done, click “Save and Close” to proceed.