While you can use 1 image for everything, this is a real waste of bandwidth and load time for the pages ...
/images <--- main image
/images/medium <--- 1 medium image for the Product _info page
/images/large <--- Large Image for when clicking on the image on the Product _info page
/images <--- additional images
/images/large <---- additional large images
This allows you to have small images at 1 to 10k for the product throughout the shop and then the larger image for when people want to see the larger image ... it saves a lot of time and looks better as the files are not squished or stretched ...
Loading the image files with FTP is used for all of the images except for the base image of the Product, which can be loaded when adding/editing a product ...
You might search the FAQs on images to learn more about the images for products and how to use them ...