I always got some issue when designing the cover for my Facebook Timeline profile page. It is easy to make the cover image, just upload picture or photo, and align it on the Facebook.
But, to make the perfect cover, it must be well prepared. First, to ensure no area that will cropped by Facebook, set the image dimension exactly at 851 pixels (horizontal) by 351 pixels (vertical). You can set larger dimension with the same aspect ratio, but it is not recommended since the image file size will also larger, that means will "cost more time" for browser to load the image.