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.
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.