Image Watermarks?

A few online vendors are using photos of my finished builds for their website sales.
In a way I'm flattered, but I'm never given credit for the builds or picture.
This blog is a business. Some finished rockets are sold to vendors and collectors.
If I post a build, what gives others the right to sell their product using those images?

I always email and ask before posting someone else's work.
There are a few blog posts in draft form that can't be used until I get permission.

As soon as I can figure out how to add a watermark, anyone who copies blog pictures will have my name on it. Others have used my build images, but they simply asked before doing so.

My personal images have been copied and pasted before.
My favorite was a phone call I got from a angry woman.
"How dare you tell me you love me! I don't even who you are anymore!"  
Apparently a guitar playing Casanova copied a picture of me playing the guitar and used it to meet women online! She did a web search and somehow got my home phone.
After we talked on the phone she did more detective work and found his real identity and address. A carefully worded registered letter (with a threat to contact his wife!) stopped him.

Just because you can right click and "copy" doesn't give you ownership of another person's work.
Every picture you find on a search has the origin address. Do the right thing and ask before you copy and paste.