You need a graphic---851px x 315px. If it's not the right size it will be cut by Facebook's program and sometimes that doesn't look so great. Once the photo/graphic is uploaded it can be moved around if necessary to get the photo at the best advantage.
You'll also need a square image 180px x 180px. Your logo or picture that you have been using may be something that you can continue to use. Or this is may be an opportune time to update your images and make sure that your whole front page reflects your branding and what you do.
Updating these images is the bare minimum that needs to be done. There are a many things that you can add to the body of the page and many things you can do to manipulate the information on your page to make a better experience for your visitors.