My father is a water artist. This and some related printing work is what raised his family:
http://kenfrye-artist.blogspot.com/
Yes, a few times he has had work stolen, both by scanning originals in the pre-internet age, and now by occasional web site rip-offs.
One offender was reproduced his work on T-shirts, 9x11s, and post cards. As my father was in that market and there were material damages, we had to take action (successful).
Regarding web site rip-offs, we keep the images small enough that you cannot make art quality prints. We figure we don't loose many real customers. We do attract real customers. Actually, most of the work that comes from the site is not purchases, but rather commissions to paint homes and boats.
I had a portion of a rock climbing guide I wrote ripped of once years ago. The reality was I wasn't going to sell enough of them that the monetary damage mattered, so I muttered to myself and put it out of my mind. I didn't like it, but it wasn't "material." I took it as evidence that the thief was a poor writer and felt good about that.
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I feel you pain, but it's not worth the internal angst. You post because you love to share. We can feel that. I do the same, though to a lesser extent and with less mastery. I do it because I love sharing my little discoveries. It's worth it.