An Online Artist's Coop for Artists who Paint on Location
In my "other life" I am a photographer, so there are a lot of my images floating around out there. Once an image posted on the internet, it is likely to roam far and wide, so you have to weigh the risk vs. reward for what you upload. Sites like Facebook strip the metadata from the image file and then make it extremely easy to share (and download!!) images. So the two things you can do are:
~ Post images at a relatively small size. 600 pixels on the longest side is adequate.
~ Watermark every image you upload ... anywhere.
If you don't know how to watermark your images, Google the phrase and you'll find a dozen different ways to do it.
We photographers have experimented with all different sorts of content for our watermarks, everything from a big © in the middle of the photo with our names below it, to our names stamped in repeating patterns across the image. Someone I know, who photographs events with lots of kids who are way too internet savvy for anyone's good, uses a watermark that says, "If you see this photo anywhere but www.XXXXXXX.com, it's been stolen!" I normally use "©www.TerriMiller.com Paintings & Photography" at about 40-50% opacity, approximately 1/3 - 1/2 the width of the image, depending on how high risk I think the image is of being pilfered. With this imprint, folks know exactly who produced the image, how to contact me, and that I do this for a living.
There have been volumes written about what to do if your copyright has been infringed. This is a good short course
It's written with photographers in mind, but the information is valid for artists as well.