I think adding a link could help people trace the image back to your website. I don't think a watermark would be the best idea as it obstructs the artwork. The only concern I have about a link at the bottom is that it is really easy to crop it out if a thief wanted to.
If you are planning on making a print or selling the image I think it would be a good idea to post it at a low resolution, so that people can see what it looks like, but can't steal it. People reposting free work without credit is rough, but I think it's at least comforting enough to know that they liked your work enough to repost it.