It's bumpmap texture.
With them, you can add bumpmapping to your textures.
In case you don't know what bumpmapping is...
This is a ball.
An orange ball.
An orange, round ball.
Balls, fuck yeah. They're awesome. Bounce 'n shit.
Wish I had one now...
...
Oh yeah, about bumpmapping...
This is a bumpmapped orange ball.
So yeah, bumpmapping adds a 3D feeling to the texture with all kind of bumps.
With GIMP, go to Filters -> Map -> Bump Map...
There you can set bumpmapping to your picture.
There are probably some other ways, too, but that's all I know... At the moment.