That's the primary purpose of most infographics these days, linkbacks for search engine juice.
I consider it ethical if there is no consideration involved--money, favors, etc. Interesting content should be able to travel around the Web if people actually find it interesting. The creator getting compensated with a little SEO bump is ok if they actually created something interesting.
That said, HN has a policy (or tendency at least) to prefer the original source and I like that too.