There's no rule here about Ouendan or EBA songs. You can try to make them accurate to the game versions "we call those "authentic") or you can make a totally different map of the same song.
There's also no rule here about using game graphics, although I'm not sure how well that would work. Depending on what you're trying to do, you might have to redraw them or try to vectorize them to get them to an appropriate size without too much blurring or pixelation. I posted some Osu-sized versions of EBA images
in this thread, and I'm sure similar effects could be used on Ouendan.
If you're worried about copyrights and whatnot, I'd say that Osu is more likely to get in trouble with music companies than with iNiS, so I wouldn't worry too much
