I basically kept my fanart for LJ and my fanart for Tumblr separate, not x-posting either of the works to the other site. The pieces I made for Tumblr were only posted there, even though I would put both my LJ username & my Tumblr name on them.
However, my fanart posted to Tumblr barely got any recognition. Once in a while I'd get a few notes, but for the most part they were ignored. I'll still post caps and pictures on Tumblr, but the fanart days are over.
I'll post my Tumblr fanart pieces here to my LJ now. I'm pretty sure they'll be more appreciated here.