Hi,
Unfortunately, the add to cart links don't support that easily.
If you're a developer it's possible by adding your one dropdown with a onchange attribute to update the product_id in the URL of the add to cart button to set in it the id of the selected variant.
But if it's in the images description of your extension, then that won't work and won't be practical.
It would be better to have several add to cart buttons, each one with the variant id as product_id in the URL of the add to cart link.
That way it doesn't require any coding and it fairly easy to add in the images description of your extension.