Anyone can create a graphic. Whether you can create a decent graphic is another matter. Your only other option is to get one created for you, which is going to cost (you can post in Custom Service/Development requests for this), or search the web for a suitable free graphic (though you'll have to make sure it's free for use elsewhere or you'll get slapped for just nicking one). Chances are any royalty free graphics are going to cost you.
Anyway you've been given the options, the rest is now up to you. Welcome to the world of being an admin on a website.