I have read the very post: HOW-TO: Image Preparation.
It's useful for me.
Thanks all the folks who help me.

Now I'd My ZC displays images in 2 sizes:thumbnail, a medium(=large) size.
I want the thumnail in 100*100 px while the medium and large size has the common size in 400*400 px.

So I upload the 100*100 pics to /images and the 400*400 pics to /images/medium.
But when I clicked for the LARGE IMAGE, it showed me the thumbnail.

I want the large and medium size share the same pic.
What shall I do ?