If you post your artwork images online, I believe it is not a matter of if it will be stolen. It is a matter of when. Because of that, I am of the opinion that it should ALWAYS be watermarked so anyone who sees the snagged image can read your name, do a web search, and find you. As far as I’m concerned, the question is how big and bold a watermark should it be. Only you can decide what it should look like and how big it should be. All-over or central watermarks are the best at discouraging theft, but they will discourage faves, shares, and purchases for the same reasons. I usually just type my name and the year in 12-16 point font (depending upon the artwork) and use a Photoshop filter to blend it in partially so it’s not so jarring. I then place it somewhere with detail, but non-central to the image. A lot of artists and photographers I know place their watermarks (even if just name text) central to the image, but even small watermarks can cause the same problems as large watermarks if they are placed such that they distract from the actual artwork.
The other thing you can do to protect artwork you post is make sure the image is small enough it is useless for printing. An art image that is 800 pixels wide is more than big enough to show up nicely on screen, but it won’t print at any size worth printing. This is especially true if you use jpg compression to reduce your image size into the mid range, which again won’t usually affect screen viewing but greatly reduces print quality.
On Fine Art America I post artwork at full print resolution even if I’m not offering prints because people only see full resolution in little squares. Yes, someone could conceivably screenprint each square and re-assemble them, but really, that’s a lot of bother, and if someone is willing to go to that much bother they’ll figure out how to get around any protection you use. Even the huge watermarks can be removed if someone is determined enough and takes the time.
Another neat trick I found out about last week can be done from Google Chrome. Go to Google Image search from a Chrome browser, open up your file browser, and drag and drop your artwork image into the search bar. It will automatically do a search looking for online occurrences of that image and derived images, allowing you to quickly and easily find art thieves. Most, if not all of the results will be your own postings, but it’s nice to know there is a very easy way to check.