I had and have the same concerns and questions you do. Personally, I figured my best work is yet to come and I thought that if I wanted to learn to do art in this digital world I would have to take some chances along the way. Yeah, I look at my settings/attachments here and see that folks are downloading my images. I can't figure out why they would download some of the little scribble experiments but whatever. In the long haul, I learn with every post of mine. The advice to not post at full resolution is probably the most certain protection. If they don't have all the pixels you do and/or don't have the full size image you have all the proof it is your work. I always sign my pieces but that is easily cropped. Also state your reserved rights or whatever along with the image.

I was watching a professional artist working on a piece and he said, "Anything I draw, I can draw again. Usually better than the first time." So, if you had to, you could recreate a map by hand and it would probably be better than your first version anyway.

It is the Internet and is filled with people lacking in moral fiber, someone somewhere is going to take your artwork and wallpaper their room or something. Are they going to make millions off of it? If they can do that with mine, I want them to contact me because for a small share I'd be happy to provide them with a heckofa lot more. Clearly they have a business savvy I lack.

I'm also 43 years old and figure I've only got so many decades left to do this. I can't afford to horde my art in the top of my closet any longer. If I get ripped off I'll be livid, but anything I create I can do better so in the end I figure they are only getting my leftovers. Time for me to step out of my comfort zone.

If you go sign up on diviantArt they have a setting to put their watermark on anything you post there. It's free but it wouldn't be the same as hosting it here and you might not get as many comments/critiques simply because some might not want to navigate away from this site.