i found a really cool site that u can make posters up to 20m square (apparently) but u can make some big enough pics.Basically you send them an image and then size it up online and they send it you in pdf format all ready to print then you print it out and copy each one loadsa times then you got some crazy sized posters.
http://homokaasu.org/rasterbator/
its best to use simple black and white pics i found