Sorry, im not 100% sure if I understand your question.

Lets say I have 10 products in the whole website. Each product has 3 images.

The full size for all three of them is 400x400px.

When you are in the product page, you can see 3 images. The 1st one or the main image is; img_1.jpg which shows at 100x100px and expands to 400x400px when you click onto it to see it.

Then below that we have two other images; img_2.jpg = 100x100 px and when you click onto it you see a pic that is 400x400px.

The 3rd image; img_3.jpg = 400x400 px and we would like it to show at 400x400px.

Did that answer your q?