If you read the language in the contract you agree to when you join many online discussion forums, you may find that once you post an image on the site some of your rights to that image transfer to the site. This is true, for instance of the site of the National Association of Watch and Clock Collectors here in the US.
As far as watermarking is concerned, I will do it if the photo in question was taken for commercial purposes on my part. Most of the photos I take for these boards are just grabs I take in window light for documentation and I don't really care enough about them to worry about someone using them. The other thing about a watermark is that...most of the time I could take it out in Photoshop in a couple of minutes if I really wanted to.