To some this may be a big deal but I can’t be bothered. If someone really wants to use an image of my painting then I hope that they will pay me something. However if they don’t, I would still be very flattered. If I thought they made a ton of money on it, then I would try to find a Lawyer who would take the case on contingency. This might be easier to do in the US than in Canada. Certainly if I copyright my work I would be in a better position in a lawsuit but I don’t want to be in that mindset. I could put a copyright symbol on everything but legally I don’t think it makes a difference.
Ultimately I just don’t want to think about copyright. As a graphic designer it was always in the back of my mind. As an artist I’d rather think about my next painting (or what the present one needs).
In Canada I painted it so it’s already copyrighted.
Here is a much more detailed treatise on why you should copyright art in the U.S.