Hello,
What you asking is impossible, i am guessing you have some knowledge on coding so i will explain why.
Try to take one image online and drag it into browsers link bar, or on any text field, you will notice that what you dragging is thats image link (link that you would go to when you click on image) and not url of image itself. It is very very rare case where you would click on image online and it would take you to its original url on server (like when you copy image url with right click), then your idea of drag and drop would be possible, but it just isnt the case on 90% of images online.
Only solution is to make text field inside form, where you can paste URL of image after copying, that i can do.
Or to make browser extension that would take all info on image (including url) and send it directly to your website and upload it, this i can do too but its much more complex job.
If you have any questions contact me via pm.
Kind Regards,
Kristijan S.