The best I have found is the clip rail system.
It's a rail you place at the top of the wall where the wall meets the ceiling and then use wire or nylon thread to hang the pictures. The one we have is fully enclosed so you can't see the hooks and stuff.
You can see details at:
http://www.artestuff.com/index.php?c...7512a666e3a4b7
or just Google clip rail.
Can't remember the name of the gallery/shop but it's located at Kowloon Tong, I think it's on the 2nd level.
There office is at:
Minitrade Limited
15-33 Kwai Tak Street
KWAI CHUNG
HONG KONG
Tel: +852-2409 6628
Fax: +852-2409 5567