Please don't do this.
When I make fanarts for LJ, they're for Livejournal. Likewise, when I make fanarts for Tumblr, I post them there. If I want to share these pieces between the two sites I do so.
I'm flattered that someone liked the header so much to post it on Tumblr, but I dislike credit not being given to me for the work I did. Therefore, if you're going to re-post something of mine here on LJ over at Tumblr, or Twitter, or wherever...please ask me first, or at least credit me in your post using my LJ username.