The biggest issue with images online is commercial images that are copied and used without payment. Chances are most of the people who want to rip you off are going to find a way to do it. Unless you are able to devote time and money to tracking these people down who are making money off of your work (by re-selling) then it can be counter-productive (ie a GIANT waste of time and energy). All of the images I've seen protected in some manner I've been able to use or edit. So far nothing has prevented me (this I say as an interesting exercise in seeing how strong protection methods are NOT that I routinely rip off others work for my benefit). With the massive proliferation of image libraries and accessibility to digital cameras, images are easy to acquire legitimately. Where once large libraries charged hundreds for an image available for print size, you can get Joe Bloggs for $15 at iStockphoto with something just as good. Even the premium libraries have been forced to acknowledge their dominance is over by dropping their prices.
Most people who are going to pay are happy to pay to licence images. Make it easy to get paid and clear what the restrictions are and enjoy your digital content going out into the interwebz. Digital files are easily copied and don't actually cost you anything other than a few bucks for hosting, so unless you want to spend your time in enforcement mode, just make it easy to get paid and let the good honest people pay you. It is likely that is all you were going to get in the first place.