It doesn't matter if they add or delete some text or photos, it doesn't matter if they flip photos or drawings over, it doesn't matter if they change the colors, or make any other changes. THE MATERIAL BELONGS TO YOU.
Agreed. Technically they will have formed a derivative work, which both you and they will own. A third person who wanted to copy it would have to get permission from both of you. They do get some rights from their contribution, but you still keep your rights from your contribution.
Derivative works are still copyright violations if the material is used without permission. This can be fairly fuzzy, depending on how much of a work is incorporated into the derived work.
The concept that you can simply change a few elements, or add to a work, and dodge copyright violations issues is false. I'm not picking on the post quoted, but responding to a few other posts in the thread on this point. I think what's happening is confusing trademark and patent issues with copyright. All three are distinctly different as far as the legal ramifications.