Pictures posted on-line do NOT become public domain unless the owner specifically states that as fact.
If the picture is not yours then you have no rights to use it period.
As the owner you can sue anyone who uses your pictures without permission, but it becomes a balance of "damages" vs legal costs. If the scum bag who stole the picture is just re-posting it on-line as their own there isn't much you can do. However, if it's a company who is obviously on-line selling stuff then that's something worth looking into. Talk to a lawyer for sure. If the company is making money then they are in fact a good target for such a lawsuit.
I have had several "copyright protected" works stolen and used illegally and after a few conversations with the people who stole them and the threat of my lawyer being the next call they were promptly removed from use.
Always protect what's yours!