With artwork it is a little more complicated. I put a lot of time and effort into what I do, and the person who I'm working for has paid money so they can use the work (though I usually retain copyright). When someone else takes it without asking, it's bad form. It's not always done with malicious intent. Some people are genuinely unaware that just because the artwork is out there - on the internet, on a poster, in a book - it can be used, for free. The work's been done, someone's possibly been paid, now it's public property. Well, that's not so, I'm afraid.
Making the discovery gives me a couple of feelings. I feel bad that someone thinks my work can just be used for free, without any credit or remuneration. I also feel slightly flattered that they think it's good enough to use. I don't call in the lawyers or rant and rave and have a go at the person or company, when discovered. I usually write a pleasant email drawing their attention to the fact that they have done something that is not morally correct, and that I feel just as bad having to point this out as they probably do about hearing it. And I don't feel good doing it, but then again, I can't let it pass either.
There's a couple of examples below. My Oliver logo (in black and white) was used on what is actually a nice colourful poster. It's been flipped and Oliver has been given a little peaked cap, but it is my logo. Mine was designed in the late nineties, well before the recent Oliver film which has also used a similar 3-person silhouette (but quite different). I'm less worried about the more amateur copy of my Sweeny Todd poster, partly because they've gone to the trouble of redrawing it (and no, that isn't normally a viable excuse!), and partly because it was for a very small amateur production (I didn't write to them about that one).
Artists and writers don't always just 'knock these things out', which is often assumed. I have a talent for drawing, but I still work my soul out at the drawing table, and that talent is the result of years of sitting in front of a piece of paper and practising and practising. The original Sweeny poster I actually drew and painted twice, as the original was sold at auction to help fund the show. I was then asked for another to put in the lobby - it was never actually used for publicity purposes in the end! I spent many hours on them. The Oliver logo may look pretty simple, and it is, but it had a huge number of rough sketches going back and forth before the client was happy.