The cover image is the first thing your Facebook friends see when accesing your profile (timeline) page. It is also one of the key elements that you can customize to make your profile page stand out.
To create a cover image on your own you will need to create a image that is 850 by 350 pixels in size. Anything smaller or larger will be adjusted to this size (either by crop or stretch) so it’s best if you create it in the optimal size from the first time. You will also need to take into account that the profile photo will be overlayed over the bottom left corner of your cover photo so don’t place there any important elements (text, key images) otherwise it might be covered by the profile image. For a more creative effect you can make the profile image integrated in the cover photo.
Here are a few nice examples: