While the product images are all added and managed in the Products admin, you access the theme's controls for product images through the theme editor.
For information on the optimal product image sizes and other admin components, check out Shopify's article:
Theme editor settings for product images
Navigate to the theme editor settings by going to Customize > Open a product page in the right-side preview or select Product page from the page dropdown > In the left-side settings, open Product pages
Enable image zoom
For product images over 1600px in width, this option will activate a zoom feature for customers to magnify the image for a closer look. Activate this feature by selecting Enable image zoom.
Choose a position for thumbnail images
Select the Product thumbnail position from the dropdown menu. This option controls the additional images that appear alongside the main image. Show them to the Left of main image, Right of main image, or Below main image. To display the additional images in full and stacked vertically, select No thumbnails, display as list.
Variant selection changing the main image
The No thumbnails, display as list option will not change the main image when a variant is selected. To have the main image change to variant-specific images, choose one of the other options.