It is not theft: the act of stealing; specifically : the felonious taking and removing of personal property with intent to deprive the rightful owner of it.
Notice it says with the intent to deprice the rightful owner of it. That is why people are saying it is not theft. Piracy: the unauthorized reproduction or use of a copyrighted book, recording, television program, patented invention, trademarked product, etc.
There is a difference and that is whay people are trying to get you to see. With theft you have taken something of value from someone that they can not replace. Theft actually removes something of value from someone, causing them to directly lose money. Piracy allows someone to enjoy something without depriving anyone else from it. It is not the same as taking it.
Your example is the equivalent of piracy. Yes, I would be upset, but I would still be able to sell those posters you got scans of. Would I allow you to do that, no. If you did though I wouldn't claim a loss of $X.XX on my taxes due to theft. I would just make sure you don't get a chance to scan them anymore.
As an aside, it looks like cell phone cameras caused Disneyland to not care about their rollercoaster pictures anymore. We went there and the stands to purchase them were empty and the monitors mostly were broken. Why? People would take pictures of the screen with them on it and not even think about buying it. It wasn't theft, but it was wrong and could be seen as piracy.