Digimark is good - i use it too.
If you are concerned about people stealing your images - you need to look in to copyrighting your images with the Library of Congress. There are great tutorials online and if i get a chance, I'll post some links.
If using someone else's pics - you must get their permission or they can and sometimes will come after you. I have a friend that will sue for a minimum of $250,000 per image per use and since his son is an attorney, the fees are not an issue. He always wins. If you remove someone's copyright mark or digimarc - it increases the fines. I think its a minimum mandatory $25,000 if the image is copyrighted through the LOC.
If you are using your own images, it depends on the subject and how it is used. If it is newsworthy - you can get away with posting editorial content as needed. If you are showing a face - any face - its best you get a signed and witnessed model release before you post it. Even after getting the release, its best to talk to your subject to see if they are ok with your intended usage. Its best to keep peace than fight out your rights in court. Only the attorneys win that jackpot.
If you are shooting a product that you created - its likely you can post it at will. If it is a product of another company, you had better get a release. If you are posting an image that contains a recognizable building in the background - you better get a release.
Familiarize yourself with all of the companies - from Disney World to Porsche and everything on both ends to the middle that you cannot shoot. Wanna shoot a NASCAR car? Look at all the decals and tell me they aren't each copyrighted or trademarked.
Its a big deal - and while its new territory to technicians with a camera - its a well understood issue of rights to those holding the patents, copyrights and trademarks.
Never post an image of a patient or person without their written consent or a full understanding of your own exposure.
