As the customer navigates through you store, he/she will encounter three different sizes of images (small, medium, and large). If you upload only one image to ZenCart using the admin interface, it will still work. However, the image rendering will not be optimized.

For better performance, you can upload different image sizes to the Zencart. The concept is explain best here and here. And for more technical instructions, see FAQ on How to Add Multiple Images to a Product.