The new Facebook timeline, introduced on Thursday shows a large cover photo right up front; but lets be honest. If you have troves of tagged photos at parties with the large variety of unfavourable faces that I have, you're going to want something that looks good. So there's two ways to do this:
Make your own: Fire up Photoshop/Paint/Inkscape/etc and make something awesome. Pixel size is 850x315px.
Take one off the web: As quick as timeline was announced, a site dedicated to providing timeline cover photos is already born. Go to http://facebookprofilecovers.com/ and take a look through. Granted, it's all environmental landscapes at the moment; but hopefully the database will increase soon