Shift print-screen, ctrl-V in image eding software...  Or... Browser taskbar, tools, page-info, media tab...  Or... Pearl Crescent Page Saver plugin... (These methods are also for legitimate purposes btw. One doesn't always have the same media on separate computers when working in group projects involving web design.) If somebody really wants an image there are so many ways. Once it's viewable on a computer from the web, it's pretty much fair game in that regard. The right-click thing is just a way to separate fools from their money.

If you're seriously attached to your images, do the watermark thing small-size things. But some people will be put off by that if it's too obtrusive. Subtlety is probably better in that regard. (And some people will still be idiots and steal those, as occasionally seen on blogs like PhotoshopDisasters.)

Alternately if you have the money behind you, there are services that do image lookups or data-embedded watermarks and then pursue copyright enforcement. (You can also use the tin eye reverse image search if you want to put in your own time pursuing such affairs. But that way isn't likely the most reliable and the service is still experimental.) Still most images that are stolen and used commercially are often done cross-jurisdiction because they don't suspect somebody in one country finding an image used without permission overseas. That makes pursuing infringement cases even more tricky. So good luck with that method too.

Only way to win is to keep an image offline in regards to the internet, unless you want to make it tiny and plop crap (an ugly watermark) on it. Or you can take the care-free approach and not be in it for the money and find that somebody wanting your images is a little flattering. (I still wouldn't hesitate to call out people claiming your work as their own though. That's a little different than them borrowing an image but not saying anything.) But I guess the economy has tightened up things a bit these days and some people are less willing to be loose in that regard.

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.