When a product has multiple price points (for example Small / Medium / Large), this setting requires a separate photo to be uploaded for each price variation, rather than showing the same image regardless of size selected.
Go to Dashboard > Products > Product Settings and find Use Images for Price Variations under Advanced Options.
Showing a genuinely different photo for a Large arrangement versus a Small one sets more accurate customer expectations, particularly for products where the size difference is visually obvious.
Once turned on, you will need to upload a photo for every price variation on every product, so only enable this if you can commit to keeping those images complete.