Hi Ratan,
I know it is has been a while since you posted this, I ran into the same issue in our Wagtail set up and have since solved it for our use case (which is an internal intranet application).
I have detailed the basic version of our implementation here:
Our actual full set up is a bit more complicated as we want to handle how the image is sent via emails and shows up in form submission listings, plus we want extra info added to the image title when created. Nonetheless, I hope this helps you out.