Koichi's sketches have increased in value by as much as a million dollars at auctions.
There used to be lot of graffiti along the underground passages at Shinjuku Station a long time ago, but we rarely see them now.
Spray-painting graffiti is already a bit old-fashioned. The newer artists create works of art by scrubbing the dirt off the wall.
When writing just happens to "fall" on the wall (whoops, wasn't me who did that, how strannnnge) it's writing that's not supposed to be where it is. This kind of writing is graffiti.
This is a jukugo word, which usually means on'yomi readings from the kanji. 書, however, is kun'yomi in this case and also has rendaku. So just remember that it's read as が for this vocab and you'll be fine.