Cover images must be at least 399 pixels wide and may not contain:
Price or purchase information, such as “40% off” or “Download it at our website”
Contact information, such as web address, email, mailing address or other information intended for your Page’s About section
References to user interface elements, such as Like or Share, or any other Facebook site features
Calls to action, such as “Get it now” or “Tell your friends”
To get the fastest load times for your Page, upload an sRGB JPG file that’s 851 pixels wide, 315 pixels tall and less than 100 kilobytes. For images with your logo or text content, you may get a higher quality result by using a PNG file.
More info here: http://www.facebook.com/help/276329115767498/