Margot Potter, aka The Impatient Crafter, just wrote the best darn post I've ever read about understanding crafters' intellectual property. Here's a taste of Margot's wise words:
"Anything that I post here that isn't attributed to someone else is under my copyright...You may make it for personal use, you may not make it and sell it on Etsy or at your local craft fair or anywhere else for that matter. You may not turn it into a class and make money on it without my explicit permission and a percentage of your fee! Now you might do it anyway and I might not catch you, but it's a karma thing baby." (Full post here.)
I often think about what might happen after I post images of my projects up on the internet. For example, a number of people have added this mirror as a favorite on Flickr. In a perfect world, they'll comment on the photo. But will anyone make the same mirror and blog about it or submit it for publication, claiming the idea as their own? Sure, people can come up with the same idea at the same time or put their own spin on a particular project, but if you saw it on someone else's site first then you owe credit where credit is due.
I recently started putting watermarks on some of my images, but I don't have Photoshop at home and haven't been able to figure out how to do it with iPhoto (anyone know?), so that has slowed me down. I think I'll take Margot's advice and add a copyright notice to the images I post on Flickr, as well as in the sidebar of my blog. It pains me to think that someone else could steal one of my original projects and publish it under their name.
One thing I have been doing to keep track of stuff like this is set up a Google alert for the terms "sassy crafter" and "the sassy crafter." Then I automatically get emails when these terms appear on pages. Of course, this won't notify me of anyone who lifts an image without attributing it, but it's still pretty handy to see where you or your work might be popping up.