This bug has been annoying me for a while too. It's always been such a useful tool...
Workaround: If you do a quick export (CMD-E) to a Temp folder (I just have a Temp folder as a favourite in Finder to put throw away stuff in) then the drag will work. It mustn't be storing the image as a temporary file properly since an upgrade somewhere in the last year.
Hopefully they do an update soonish as I can't really be bothered finding/learning a new tool...