Those textures seem random and very evenly distributed.
closed as off topic by Philip Regan Aug 11 '11 at 11:39
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The basic technique is to create a background color or gradient, add a 50% Grey layer above it with its blend mode set to "Overlay" and add some noise to that layer using whatever method your program provides. In Photoshop it's a filter, and I seem to recall it's much the same in the gimp. Adjust the amount of noise (which should be monochromatic) and opacity to suit.
Use the noise filter to create a low opacity overlay/burn texture layer. Blamo!