Maybe we can set guidelines for such situations:
I think guidelines would be very good.
As much as I agree with you, it's against the law. You cannot just pick up a car that's in your street for two weeks and the house behind it looks empty.
You can't just take a set of pictures from the Internet, wrap them, and give them to others.
Lots of artists do not mind if you take their work and use it. However even today there are a number of artists here that do not like their graphics being used without their explicit permission.
We respect that wish. Does that end the moment they close the door behind them from here? What about the work they already published here under the assumption they would have keep the rights?
Owen will delete it all?
Then there is the trouble of how far do you go. A sprite published for TTDP, can it be used for OpenTTD. Maybe I see a nice eye-candy sprite for my FreeRCT project... May I make a copy?
How would the license work. Can the new person make it CC-BY-SA, or GPL, or BSD-style, or commercial?
I think the 32bpp-tar project showed how complicated it is to do anything like that afterwards.
I don't see a clean way out of this. At some point you're making assumptions about the intent of the original author, and without him around to express his wish, you're inevitably making a wrong decision at some point. The question is then do you always err on the safe side, or not.
I find it hard to believe someone who drew graphics in 2010 but hasn't visited since will care that someone else has picked them up.
I find it hard to believe that artists want their work to be used by everybody, yet don't use bananas, the main-stream service for distributing game content for OpenTTD. Yet, as you know, some artists make that decision. I don't see why such a thing couldn't happen with random sprites published 5 years ago.
Talking of guide-lines, you could make it a forum rule "anything published here has FOO license, unless otherwise stated". I don't know how much legal ground that gives you though, and of course it wouldn't hold for anything posted before. Perhaps a better solution is to let each artist decide what license the sprites have when they publish sprites. That could be in the message, or in the image file itself (but not in the signature, as that can be changed afterwards).
You'd probably have to explain some things about it, and make a choice for artists who don't care. ("If you don't want to know about licenses, use FOO license.", or some flow-chart "can others use your work?" yes/no)