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Simplicity is Complex by John Maeda
You can create a tile that repeats and serves as the background for the entire image header. The edges of the tile should meet, such that the image can be repeated without any visible borders or breaks.
Your header image can then place objects on top of the background, leaving other areas transparent so that the background image shows through. The header image should be the same width as the background tile.
John Maeda's theme "Simplicity is Complex" is a great example of this technique. In the screenshot, the red lines mark the edges of the tiled graphic he used for the header.
World-renowned graphic designer, artist and computer scientist, John Maeda has been a pioneer in using electronic media as a tool for creative expression - from his groundbreaking work on interactive motion graphics to his research on helping people simplify their lives and work in an increasingly complex time. John holds the E. Rudge and Nancy Allen Professorship of Media Arts and Sciences, is the Associate Director of Research at the MIT Media Laboratory, and was recently named as the next President of the Rhode Island School of Design (RISD). He has written four books including The Laws of Simplicity.