I saw this annoying bug for the first time today.
The issue:
Could not drag an image from Skitch to another app (tried Slack and Gmail). When the image was dropped, it was simply a white image with the name 'Blank.jpg'.
The solution:
In Skitch:
Take a screenshot and insert an annotation (I created a Skitch annotation Box).
Delete the annotation.
Drag your image to an outside program.
This seems to have resolved the issue for the time being.