There are also some wonderful street artists along the avenue of the stars promenade who sell original artwork and have some really nice paintings and photography.
Also there is a really cool small photograph gallery at the star ferry terminal pier in Tsim Sha Tsui as you go up the stairs towards the central station on Kowloon. It's called the Lawrence Lai Gallery and he has some gorgeous matted photographs, every imaginable size of Hong Kong and is not too expensive.
Check out the links and shopping pages on my site at
http://joshongkong.tripod.com for more info.