The information and guidelines below come from the Facebook document How should I choose a cover photo for my Page?
In summary, you want to “upload an sRGB JPG file that’s 851 pixels wide, 315 pixels tall and less than 100 kilobytes.”
Remember that your page profile image will cover a square area in the lower left of the header image.
If you don’t have a graphic design program, you can use PowerPoint or Keynote (Apple) to create an image using photos, graphics, and text. It may require some experimentation to get the layout just right.
Choosing a Facebook Page Cover Photo Image
Use a unique image that represents your Page. This might be a photo of a popular menu item, album artwork or a picture of people using your product. Be creative and experiment with images your audience responds well to.
Cover images must be at least 399 pixels wide and may not contain:
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Price or purchase information, such as “40% off” or “Download it at our website”
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Contact information, such as web address, email, mailing address or other information intended for your Page’s About section
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References to user interface elements, such as Like or Share, or any other Facebook site features
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Calls to action, such as “Get it now” or “Tell your friends”
To get the best quality image and fastest load times for your Page, upload an sRGB JPG file that’s 851 pixels wide, 315 pixels tall and less than 100 kilobytes.
All cover images are public, which means anyone visiting your Page will be able to see the image you choose. Covers must not be false, deceptive or misleading, and must not infringe on third parties’ intellectual property. You may not encourage or incentivize people to upload your cover image to their personal timelines.
Learn more about Pages guidelines.