We recommend that you chose one shape for all your product images on your site. Squares work best for most things. Rectangles (portrait) works well for clothing if your using a model or mannequin. Rectangles (landscape) works well if your have large items like sofas.
*Note: Many stores sell a variety of items, clothing, jewelry, furniture etc., but still use only one shape for all their images. Below are a few examples.
Shopify Links and Cheatsheet:
2048 x 2048 is the largest Shopify allows for product images. Best to upload the largest. Shopify will re-size the master image to the sizes listed below.