Hi,
The naming has changed over the years on Virtuemart. But the functionality is still there though.
You should pick, String or Image as your custom field type, and use the "Image Alt-Text" field in the picture's description section to enter the custom-field's ID.
MultiVariant works based on "child-product" and it is much more complex than it looks. Beside, with child-product / main-product setup, no need to setup a attribute<->picture matching. Because child-product can have their individual product image, so the image will be swapped alreadt. But they are hard to setup though. You have to create child-product for every single attribute. String and Image type custom fields are much easier and efficient.
Regards.