I'll do the usual crazydrummerdude intro, by telling a story.
At a shooting competition last fall, a team member hired a photographer (girlfriend of his roommate). Previously, I was essentially the teams photographer in between my own competing. While I was running the registration table in between my skeet and trap shooting, she arrived. She laid down her stack of "cards," an excellent 3x5" picture with her information, pricing, etc on it. The picture was mine. I took it. I was there the previous winter trudging through the snow to get some shots of my teammates. It was my camera. The original is on my computer with all the related pictures. I don't know her. She didn't ask. After confronting her (boy, was she surprised at how small of a world it was!), I took the stack.
So, what I'm getting at is; Assuming I have access to the common photo editing software (I've actually never edited a single picture I've taken), what method can I use to add a professional looking watermark to my pictures?