I would have to say that Digimarc
is probably the oldest and most well known company for digital watermarking of photos, video, audio, PDF's, etc .
They should be around for quite awhile, since they have contracts with a number of states for providing secure drivers licenses, as well as some pretty big stock photo companies.
Paintshop Pro 7 even came with the plugin to add and read their watermarks, which should give you some idea of how long they have been around as a respectable company.
Their digital watermarks are not visible to the naked eye. They also allow a visible marking as well. Their hidden watermarks are supposed to resist manipulation, so they are not supposed to be easily removed from images. They also have a tracking service so you can see who is using your images online. Corbis
uses their ImageBridge
service to protect their images.
You have to pay for a subscription to get an ID from them to add your watermark to an image. Offsite backup and secure sharing is also part of the deal.
If that service is a bit more than you had wanted, they also have MyPictureMarc
which is a little less heavy duty, but offers a lot of the same protections as the ImageBridge service. It's good for collections under 5000 images. They have a few different options packages ranging from $79/per year to $499/per year, depending on the number of images you want to protect and which services you need.