Just wanted to add to this while looking for an answer on this myself. I was previously on Mac 10.12.6 Sierra which was working flawless, but when upgrading to 10.14.6 Mojave this issue started occurring where dragging a screenshot from Skitch to anything else would often provide a blank screenshot. On testing further however I noted that if I change the name of the screenshot in the bottom-left corner of Skitch before dragging this, the dragging feature would work 100% of the time. This is at least a workaround for now, though it would be nice to see this resolved.
If you need any more i