To put a picture on your header click customize in your toolbar. Then in the header area there should be a link that says edit. Just click that and it will take you to a place where you can add a picture. I'm not sure if all of the templates allow pictures, so if you can't add one, you may need to switch templates.
Then you just find a picture on your computer to upload and there you go. Sometimes you have to click shrink to fit for it to work right.