Sorry, I misunderstood you earlier. Now I understand (I think).
You need to look for 'tiled' images of camouflage. They can be joined up many times to male a big pattern. They are also known as 'seamless tiles', and they are made so they can join together without the join showing. You can join as many as you want and Gimp will join them, I'm sure.
Seamless tiles (tiled images):
https://www.google.se/search?q=tiled...NITNswbQ04D4Cg
more:
https://www.google.se/search?hl=sv&c...HcrhtQbK8YHAAQ
I know how to do it in Elements, but not in Gimp.
These might help:
http://superuser.com/questions/38439...-tiled-fashion
http://gimptips.com/articles/automat...le-images-gimp
Videos:
http://www.youtube.com/results?searc....1.0RqcMD2yYHY
Google for more information on it.
I hope this information helps you now.
Cheers
major-setback