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.
Example Sites:
http://www.sundancecatalog.com/category/furniture+and+decor/new+arrivals.do
https://www.onekingslane.com/homepage
http://www.potterybarn.com/shop/outdoor/all-outdoor/?cm_type=gnav
Shopify Links and Cheatsheet:
http://docs.shopify.com/support/your-store/products/what-size-do-my-product-images-need-to-be
http://docs.shopify.com/support/your-store/products/how-do-i-make-my-product-images-the-same-size-on-collection-pages
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.