In the scope of a couple of days, several people have seen their Facebook cover photos disappear without explanation. The issue appears to be a move by Facebook to aggressively crack down on images that are considered promotional.
The exact reason why Facebook removes each cover is a mystery, since the user is not informed, except by the glaring empty space where the photo used to be. It could be due to a copyright violation or that the photo was deemed to be “promotional.” Although Facebook removes the photo from the cover position, it doesn't actually delete the photo itself.
To ensure your cover photo stays put, take a minute to check out the full guidelines:
All covers are public. This means that anyone who visits your Page will be able to see your cover. Covers can't be deceptive, misleading, or infringe on anyone else's copyright. You may not encourage people to upload your cover to their personal timelines.
Covers may not include:
i. price or purchase information, such as “40% off” or “Download it on socialmusic.com”;
ii. contact information such as a website address, email, mailing address, or information that should go in your Page's “About” section;
iii. references to Facebook features or actions, such as “Like” or “Share” or an arrow pointing from the cover photo to any of these features; or
iv. calls to action, such as “Get it now” or “Tell your friends.”
You can see a full list of Facebook Pages terms here: https://www.facebook.com/page_guidelines.php