Under Image you can choose the Asset to display in the selected Image element by clicking +.
If you don't have the image you want to use uploaded as Asset, you can upload them by clicking + and then clicking Upload.
The Sizing selector lets you choose between:
Fit: Showing the entire Asset in the Image element.
If the Image element has a different ratio than the Asset, it leaves part of the Image element empty.
Fill: Showing as much of the Asset as the size of the Image element allows
The Image element centers on the middle of the Asset and cuts off what it cannot show.
Images in eCommerce elements
The Design panel shows different options when an Image element is part of an eCommerce element.
You will see the Content section showing the selected product and a Change source button to change this selection.
The Content selector allows you to choose whether the Image element shows:
An Asset you upload to Instant (Static), or
An image associated with the product/collection in Shopify (Dynamic).