Yep, this is easy to do. Just edit your product, select "edit all" in the inventory section that shows your variants, then next to each variant you'll see a box that looks like an "upload" arrow - click this and you can select one of your existing product images to represent the variant. I don't think you can select an image that isn't already added to the product photos though, so you'll need to already have images for each variant on the product page.
I should point out that the drop down selection box for variants tends to end up a long way from the images unless you have a very short description, so with longer description sections it's actually not that helpful, as the image that's being displayed when you change the variant isn't on screen... so there's that 😉
It'd be nice if the product pages could keep the product image on screen and just scroll the rest of the page content under/next to it - maybe that's possible, but I don't know how.
