how to handle several images in a table?

[lapoleon <linzhichu00 mails tsinghua edu cn>]

I creat a table which contains several images,and when I "click" on an 
image,a user-defined callback takes action.
My question is how can I identify which image was clicked?(because I 
don't want any image to be clicked more than once,so I have to know 
which has been clicked)
Any clue on that?