How to make the medium and large size share the same pic ?

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 100100 px while the medium and large size has the common size in 400400 px.

So I upload the 100100 pics to /images and the 400400 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 ?

It won't work the way you are asking for.

The base image will display in listing pages and wherever it can't find the medium or large image it is looking for.

The medium image will display in the product info page.

The large image will appear when the product info image is clicked.

You have nothing in the /large/ folder, so it uses the small image. If you have the same size image in medium and large, there is no point to clicking for the large image.

Thanks for your quick reply!

Now I think it's OK to disable popup window for Large Image if the medium and large image have the same size.

But I wanna know if the thumbnail and medium image have the same size, then how to avoid uploading the thumbnail images to /images/medium while the medium image share the same image with the thumbnail ?

If you don't upload an image to the /medium/ folder, Zen Cart will use the small image. You don't need to do anything else.