Yep. That’s right folks, the 20% text rule in Cover Images on Facebook is now GONE! KAPUT! OVER!
The Page rules were updated Monday 1st July 2013
The 20% text rule STILL applies to advertising though, and that includes images that you use for Sponsored Posts that inlcude links. If an image is included in that link it MUST have less than 20% text (Facebook have given users a new tool so that they can change the preview image before making the post live). Make sure you do this as Facebook will quickly suspend adverts and Sponsored stories that break these rules.
Here’s a link to the New Page Guidelines in full.
And one to Facebook’s Grid Tool for Advert Images (really handy!)