I use Pic Monkey to create a collage of my cover art. An example can be found HERE.
Pic Monkey is SO easy to use…
Click on Collage (top middle of the picmonkey page)
Upload your photos
The photos you uploaded will appear on the left hand side
Drag and drop the photos to the middle and PLAY
Play with the size of the collgae (remember to click the little lock button next to the dimensions so that you can change both)
Click on the various options to the left
Click on the layout button – there are various layouts including a FB header one.