It doesn't seem to have been fixed YET, though if someone as you said transfer alot of pictures in and out of wippiespace and notice that the .tgcache takes up alot of space then please PM me about it for the moment and we will cleanse it manually. Include wippiespace-name and your customer number.
Have a nice day!
*EDIT*
Well I heard with the technicians and the .tgcache-folder doesn't add to the users quota. So even if you have 500mb of thumbnails there you still have your 500 MB left of the G-Lite or 50GB left of Big-G

*EDIT*