Quote: "That's not a texture, it's a picture of a piece of fabric I found on the internets."
EDIT: Just read your first post...
Quote: "make smaller details on the camo, or am I gonna have to do it manually?"
I used to use Gimp, I use Photoshop now, I think you could first to Filter, Render, Clouds, Plasma. Then reduce the saturation all the way... Then make a new layer with the bovination tool or whatever to create the camo on top of the clouds. That's the only non-manual way, you could manually go in and use the dodge/burn and paint tool with low opacities to add extra detail
EDIT2: And then if you wanted it to look like cloth, I think there is a filter called 'clothify'.
-PwP