• Cover photos are
  • 851 pixels wide and 315 pixels tall (less than 100Kb for fastest load time); jpg or png (better for text).
  • public
  • Minimum size is 399 pixels wide and 150 pixels tall; if you upload less than 851px x 315px, it will be stretched.
  • NO 3rd party advertising*
  • Can NOT  be deceptive, misleading, infringe on anyone else’s copyright or be in violation of the Pages Terms. 
  • NO encouraging people to upload your cover photo to their personal Timelines. 

From the developers group policy:

You must not incentivize users to Like any Page other than your own site or application, and any incentive you provide must be available to new and existing users who Like your Page. 

[and]

You must not obscure or cover elements of our social plugins, such as the Like button or Like box plugin.

More
*https://www.facebook.com/page_guidelines.php

Dimensions
https://www.facebook.com/help/www/125379114252045

https://developers.facebook.com/policy/
https://www.facebookbrand.com/dos-donts

Using Facebook graphics/branding in your products & sites
https://www.facebookbrand.com/

Rules for Ads
https://www.facebook.com/ad_guidelines.php

make your cover fancy here;
http://www.cio.com/article/703359/5_Websites_for_Cool_Facebook_Cover_Photos_
http://coverjunction.com/
http://www.timelinecoverbanner.com/

http://hughbriss.com/google-cover-template-that-works-for-all-monitor-resolutions/

See which pages you’ve been invited to
https://www.facebook.com/pages/?category=invites