Or what to tell another artist if you see their art reposted.
If the piece was posted by you onto tumblr before the work was reposted, contact tumblr using firstname.lastname@example.org
Reposting work is against the TOS and staff will remove the post. This also applies to people tracing your work or otherwise tampering with and posting your work without your permission.
If it’s a piece that has been posted from an artist not on the site and is either unsourced or the artist has requested their work not be reposted, send a message to the artist and link them to http://tumblr.com/dmca
Reposters will sometimes remove the post from their blog if they are contacted, but this does not remove the post from the site and it will remain in circulation.
Staff can and will remove the post completely for you, so unless there’s only a few notes and you can ask people to remove it, contacting staff is a better course of action.