A website will only load as quickly as the content on the pages. Making potential customers wait for images to download before being able to browse / shop can cause them to go elsewhere.
Some website layouts may be improved by having a consistent size to your images (for example having them all square). This can give better visual impact than having various sizes of images displayed on the page.
This guide will talk you through how to resize and optimise your website images for optimum file size without losing out on image quality.