When you submit (publish) a piece of intellectual property that you have created, be it an image, a written article, or a post to a website you are automatically the copyright holder.
It's common for the website that the article or image is submitted to to state in their website terms of use that they have the right to use the piece. The website terms of use also spells out other dos and don'ts and clearly states that using the website means you agree to the terms herein. It also states that if you don't agree to please leave immediately.
I personally do not have a problem with SS labeling an image that I submit with their logo because their doing so does not mean that I am no longer the copyright holder. It only means that I have agreed to allow SS to publish my intellctual property on their website. I use the term intellectual property because technically pictures and posts that you create are intellectual property that you hold the copyright on once they are published on the web
Regardless of whether the image has a water mark, SS members that submit images that they did not create and therefore do not own the copyrights on (unless they can be confirmed to be in public domain) are opening themselves up to the rath of the original copyright holder. Just because a works doesn't say it's copyright protected doesn't mean it isn't. The world of public domain is complicated.
Usually, when a copyright holder learns of a violation a cease and desist letter is sent. Usually, no action can be taken until the cease and desist letter has been served and then only for future violations --- but not always.
It is my belief that if an author knows of a violation and doesn't pursue it the author can lose his or her right to pursue futire violations. This explains why you hear of big corporations going after small Maw & Paw shops over what looks like bullying tactics. In other words, they have to protect it or lose it.
I don't know how and where the SS website has liability but I KNOW that the person submitting the item has exposure. In essence, when you post something on the web you are publishing it on the web. It is you that pushes the "publish to the world-wide-web" button. If you are publishing someone elses intellectual property you've got a problem. The Steel Soldiers watermark is going to be like waving a red flag in front of a mad bull to the original author/copyright holder.
I'm not an attorney and I may be off base on some of the above. I've brushed up against copyright issues in my business and hopefully know enough to keep me out of trouble. One thing is for certain, copyright laws are enforceable. You can copy someone's mechanical gadget and get away with it much easier than if you copy or use their intellectual property without their approval. Be careful.
This is serious stuff that requires competent legal counsel.