The images used in my header and in my background are taken from from a beautiful map of the Internet produced by the Opte Project, a brainchild of Barrett Lyon. The original image is creative commons licensed. This means you can share and remix it as long as you attribute it correctly, don’t use it for a commercial purpose, and redistribute any changes under a similar license.
It seems to me that changing the resolution and clipping a portion of the image are probably minimal changes that don’t qualify as derivative works, but as uses of the original image. However, in the case that they do themselves qualify as works, both the background and the header are also licensed under the Creative Commons Attribution-NonCommercial-ShareAlike Generic 1.0 license.