The exact dimensions are 851×315 pixels. If you upload an image smaller than that size, it will be stretched to fit. The image must be at least 399 pixels wide, or Facebook will reject it.
This image can be just about anything you want, but Facebook does have a handful of rules on what you can’t do with your cover image:
• No contact information
• No pricing or purchase information (e.g., “20% off” or “visit our website for coupons”)
• No “calls to action” (e.g., “call us today” or “tell your friends”)
• No arrows or other indicators pointing to the “Like” or “Share” buttons
For the full list of rules regarding covers and timelines, see Facebook’s FAQ.
While the above list might seem a bit arbitrary, it makes sense once you realize that Facebook wants you to put such content in paid advertisements on the site. That is, after all, how the company makes money.
If you’re using Facebook for your business, consider using a cover image that includes the name of your company and photography or other graphics that represent the nature of your company.