Assume for the moment that you put up a large image (for the web that is). It would be 800 pixel across. On many people's screens that's 7-10 inches wide.
In other words, people can "rip-you-off" but there is no commercial use for an image that small. If you're into screen-saver sales, then it's another matter.
The image could be used commercially on the web, or on TV. Web advertising can command pretty large fees, not to mention TV. Even in print, quite a few image uses are pretty small. It is not uncommon for me to obtain several hundred dollars in licensing fees for images used in print at postage stamp size.
The main reason not to worry is that copyright infringers are not people who are going to license from you, at least for any decent fee. So you don't really lose any business to them, and if you happen to catch them, you may get a nice windfall.