Understanding images in woocommerce

Can someone clarify something for me please.
I understand that woocommerce uses 3 ‘types’ of images.
woocommerce_thumbnail – used in the product ‘grids’ in places such as the shop page.
woocommerce_single – used on single product pages.
woocommerce_gallery_thumbnail – used below the main image on the single product page to switch the gallery.
but are they three different images or just 3 variations that WC makes from 1 image that I upload?
In other words, if I want
woocommerce_single to be, say, 600 x 375 and
woocommerce_thumbnail to be, say, 390 x 390 and
woocommerce_gallery_thumbnail at, say, 300 x 300
do I create 3 different images in Photoshop, all cropped and sized as above
or
just upload one image and sett parameters in Appearance? Thanks Chris