Facebook recently announced the implementation of a 20% text overlay rule when it comes to Page Post ads in the News Feed and cover photos. They’ve actually been using this to varying degrees since at least December but are now making it more wide spread. The premise behind the rule is that any image that you choose to promote or use as a cover photo cannot consist of more than 20% text.
The way that this is being determined is through a 5 x 5 grid with a total of 25 boxes that Facebook currently uses in-house during its ad approval process. The rule stipulates that text may appear in a maximum of 5 boxes.
If you have additional questions about Facebook’s Ad Guidelines, you should check out the following: https://www.facebook.com/ad_guidelines.php