How can I upload an image for a specific image style?
Image styles are “derivatives” - they’re automatically derived from the original image you uploaded. A UI to manipulate those derivatives wouldn’t really make sense, the whole point is that they’re generated automatically. 

If you can’t change the way you’re working, as in upload the 2x version as the original, and use a 0.5x image style for desktop, and the original upload for Retina, you could always replace the generated styled image files manually via sFTP or similar. But it would become a pain quite quickly.