Most people that have a tumlbr will gladly remove it if you ask--most of us have that stipulation listed on our homepages. As far as if it's already been reblogged, unfortunately, thats just one of the breaks of posting stuff on the internet.

I can tell you, though, that lots of tumblr followers do click through to the source site if they like what they see. I did a web comic w/ a friend and we hosted it on her site and drew in 150 unique IP hits in five days--a lot of that was due to tumblr reblogs. that was the main reason I eventually started a tumblr of my own!