Rain won't even fuck with the poster. It'll stay up forever if it's on good in the first place. Any ink will do fine. I don't reccomend Prismas tho. They fade quickest. Black fades less than other colors. Drawing techniques?.... I'll let you work that one out. To make it big use a projector. Colored pictures into a black and white poster? Try running the picture through a copier and adjust the contrast to pull the shading out.