Facebook cover photo dimension is 851 pixels wide and 315 pixels tall.
1. Open your favorite image editor. In this tutorial im gonna using Adobe Photoshop CS5.
2. Set a canvas by going to file > new > width 851 pixels and height pixels 315 > click ok.
3. Open the photo you need.
4. Tweak the photo if doesn’t fit to canvas. Do not stretch the photo because it will alter the resolution. To get the best quality image, upload an RGB JPG and less than 100 kilobytes. It will make fast the load time for your page as well.
5. Save your photo by going to file > save as > PNG or JPEG. You can choose what type of format you want to use, either jpeg, png or gif. It depends on the photo to get the best resolution.