I could be wrong but I have always considered personal use of copyrighted materials to fall under the same rules as recording a TV show or putting my CD music on my computer. As long as you are not doing it for someone else or redistributing it, it is allowed.
I think this is where the line is drawn. It becomes an issue of the law when you start to promote that you are a distributor of Trade marked/Copy righted images for a profit. WHen you start putting yourself out there as a dealer of such images and when you are taking money for an item that you are not giving credit too. Or Royalties too.
I guess the point being if you are the one sign shop creating certain images and are selling the service of decals and not so much the service of distributing copyrighted images then there's a gray area that you can skate by on. These corporations aren't necessarilly going to come after you for selling some Calvin peeing on Ford knock offs for $20. But start mass producing those, start making that a solid portion of your business income and you should expect to see some lawyers walking through your door.
As for taking other sign shops images - where as it's pretty unethical and shouldn't be done. (And I wouldn't do it personally) It's one of those gray areas that makes you wonder - is said shop gonna go through the hassal of lawyering up to get other shop from using a redraw or reedit of questionable art?
I just think it would be easier to sell the art with the sign and make the customer happy to the point where you shouldn't have to worry about them going to guy across the street after they have worked with you. If your business practices are sound, your completions timely and your pricing fair, mind you I didn't say cheap or even best I said fair, then you should be able to keep most customers in your store with your/their art and their corporate brand/identity. I understand copyright issues. But I also feel that it's more about trapping a customer and saying "I made this, and although you paid for some of it through the sign or whatever you had us do, I still own this and therefore I own your business branding and everything that you have put it on and if you don't like it too bad and if you don't like me then I will make you pay out the ass to get away from me and use what I made for you". Then it is about ethical and or moral business practices. Again if your practices are good, and you make a good product people will come back. not because they have to but because you make them feel good about doing business with you.
