We're beta testing this feature right now. You won't see it in your back office unless you're part of the test group.
Your variants may not all have the same price, or if they have different colours, you may want to display another picture so that it shows exactly what the item looks like.
Edit a variant
Go to Product > Webshop
Click on the corresponding product
Then click on the "Variants" tab
Finally click on "Edit variant"
From there, you can edit multiple things so that they only apply to the corresponding variant:
image
price
supplier price
SKU
Barcode
Online availability