Get the gimp!
- the texture in the background is 'difference clouds'
Clouds and difference clouds are two of the most important elements in a graphic program.
Yes, they're simple two color cloud renderings at first, but you can keep applying more and more difference clouds and come up with crazy shapes. Then you can run filters on the difference clouds, and a simple square pattern becomes a wall of bricks because of the different tones in the clouds.
You can adjust the hue/saturation/contrast of them at any time to get lots of cool textures. You can cut shapes out of them or run them on shapes you already have.
Using a spherize filter, you can make things like planets, golf balls, or anything round, and often a combination of that + a bevel filter can help make non-sphere shapes too(like a horse or something).
The difference clouds will make almost any pattern filters, from snakeskin to fractals, seem way more convincing and real.
Here is a block tutorial i made in photoshop, its very similiar, the difference clouds are still called difference clouds, and i showed you above where to find the filters menu in gimp.
See how the clouds become blocks.
http://www.dreade.org/psblocks/blocktutorial.htm