But why not ban everyone from downloading a pic? It's safer.
It is still covered by copyright, so nothing can be done with it commercially unless you automatically assume
all people are dishonest.
But Flickr is used by the majority of members in a different way, and they don't assume their pictures have a great financial worth. They are just sharing their hobbies, perhaps classic cars, where somebody may be restoring one and needs some detail pictures, so they trawl Flickr, find somebody with a similar interest, and save a few to help with the project. It is treated more as a community by the majority. The best bet is not to put anything on that can be printed off or distributed at industry required sizes or resolutions. You can put signatures on, but I think that in most cases this ruins a good picture, or you can put watermarks on, but this can look overly precious unless the image is utterly stunning and would obviously be stolen.
But pretty well all the stories about images being stolen from the web are urban myths, or it is the same story that the same few people keep banging on about all the time.
Steve