Again, we suggest you don’t do this now, come back and play with it once you have a few pages created. You may find the pages are quite busy enough without adding background graphics, and remember that text on a picture can be a lot harder to read than text on a plain background. In fact, if you load a background image in Twenty Ten theme, you’ll find that the theme puts a white panel for the text to appear on over the picture, so you only see the picture around the edges like a frame. You can experiment with this easily enough, you can always click “Remove Background Image” to take it away again if you don’t like the result.

And now the image at the top of your pages, the Header.