If you share the work of others, it’s your duty to make sure that the creators of that work get proper credit. Crediting work in our copy-and-paste age of reblogs and retweets can seem like a futile effort, but it’s worth it, and it’s the right thing to do. You should always share the work of others as if it were your own, treating it with respect and care.
I’m guilty of not always doing this, but I want to do better. We can all do better. Especially on tumblr, where reblogging without so much as an artist’s name attached to it runs rampant.