You can choose an image from among the photos that you have already uploaded, or you can get creative and upload your own. Perhaps you would like to use one of mine?
I have over 300 images to my gallery sized for use as Facebook Cover Images. They are all available to my Members or your can purchase them individually for a small fee.
Here's what my own profile looks like using "A New Dawn"...
Where can you download them? You can browse them all directly staring here. Otherwise, if you see a wallpaper you like in my gallery then look for the 850 x 315 link under the "Member Mobile Wallpaper" links. Download it to your own computer or device and then upload to Facebook as you would any other picture file.
I will give away one free 90 day Membership TODAY (6-14-15) to a Facebook user who changes their Cover Photo to one of mine! Change it to a new one every day you want to be considered and include the hashtag #digitalblasphemy. I will announce the winner on my Facebook page tonight so I recommend you visit the page to see if you've won.