Facebook Cover photos are displayed at 851 pixels wide and 315 pixels tall. If you upload an image that's smaller than these dimensions, it will be stretched to this larger size. The image you upload must be at least 399 pixels wide and 150 pixels tall. If the photo is larger than 851 pixels, the graphic is downsized to 851 pixels wide, and the user can adjust the vertical cropping by sliding the photo up and down. Once the photo and adjustments are selected, the cropping can be adjusted at a later time.
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, you may get a higher quality result by using a PNG file.
SOURCE:
facebook.com/help/125379114252045
