BLF

I watched a documentary on (maybe) this group randomly around a year ago; I've always been into the idea behind changing up billboards, never done it myself, probably never will - but there's an awesome site, that you can check out by clicking HERE that showcases a lot of what's going on.
There's a really good History and Timeline section, where you can see where it all began. It's very interesting and well worth checking out!

“It is our belief that public art should not be illegal and forced to stay behind gallery doors, but it should be displayed in public spaces and used as a method to encourage people to question the society they reside and it’s morals.” - Robert Brown (Sydney)