So you've finally taken the plunge, hitting the "Get the new Timeline" button that forever commits you to the new chronological-style Facebook profile. One of the major differences between Timeline and the old-school profile style is the addition of a cover photo. This is the first thing your friends will see when they view your profile, and it provides an opportunity to add a bit more personality than just that smaller-sized profile pic below.
To change your cover photo, move your cursor over the cover photo canvas on the top of your profile page. Select Choose from Photos if you want to use an image that's already on Facebook, or pick Upload Photo if you want to add a new picture.
If you're experimenting with different images, you'll quickly realize that there's an optimal photo size for the Timeline cover photo. The exact dimensions are 850 x 315 pixels (8.5 x 3.2 inches), but you can also crop a larger-sized image. Once you find a photo that works, click Save Changes, and your friends will get to see a whole 'nother side.