The number one question asked about the Vanity Shopify Theme is about the optimum size for product images.
We recommend using square images, around 600px. The reason is that when Shopify resizes images it maintains the aspect ratio. For example, if you upload a product image file that is 400 × 600 (a tall rectangle), then display it using the “medium” size (as Vanity does on the front and collection pages), then it will display as 180 × 240. Similarly, if you uploaded a 600 × 400 image it would be resized as 240 × 180.
In other words, making the images square means they will not appear squished up, and making it larger will make it appear sharp in the lightbox (clicking on the image in the product page).