I would strongly recommend you use Tim's Image-Manager mod (see the link in the post above and be careful not to confuse it with an earlier contribution called ImageManager) if your store is in the early stages of development and you haven't already added hundreds of products with their associated images.

You will still need to prepare your images to the best quality you can - and you should probably ensure that they are all of the same dimensions if you want to be sure of a uniformity throughout your site - but you only need to prepare the largest version of your image and upload that when Adding/Editing a Product - you can forget about FTPing to medium and large folders ::tup