How do I add a header image to the Ultralite Theme on Tumblr?

Im new at tumblr and just know extreme basics of HTML coding. but i would like to know how to add a header image to my theme. The blog is called ‘Ashley Abstract’ and i want the image to be centered on top of it. can anyone help me?
the web adress for my blog is www.ashleyabstrac.tumblr.com
and the url for the image i want to add is http://i292.photobucket.com/albums/mm9/joebrohoe6412/Photo160.jpg

thank you!