Find Shopify's full image spec guide here: https://help.shopify.com/en/manual/products/product-media/product-media-types
According to Shopfiy, "Your product and collection images can be any size up to 5000 x 5000 px, or 25 megapixels. Product and collection images need to have a file size smaller than 20 MB to be added to Shopify. High resolution photos look more polished and professional to your customers. For square product images, a size of 2000 x 2000px usually looks best."
Our suggestion is simple, always use high-quality PNG files for Shopify product images with a file size smaller than 25MB.
According to Shopify, "The best file type for most product images is PNG, followed by JPEG. Other accepted file types are PSD, TIFF, BMP, GIF, SVG, HEIC, and WebP. Shopify's Imagery service determines which image formats are supported by your web client, and then automatically displays the image in the best format available. The exact way that images appear on your store depends on your online store theme, but all themes support adding images."
For a product listing to be successful, we recommend that you include a variety of image styles for every product to showcase their unique benefits and invite visitors into a story. Consider including the following:
It is also important to note that each image may be reformatted for various placements throughout your website, meaning that image quality is critical to look good in all sizes/placements. According to Shopify, "When you add an image, different sizes are automatically created for use in different areas of your online store theme. For example, a product image's large version might be used on the product detail page, where the customer wants to see the product in detail. The compact version of the same image might be used on the collection templates, where a larger image isn't necessary."