In the case of the Molotov Man, I do think the photographer, Susan Meiselas, had every right to feel her photograph was losing its importance by being appropriated. I also feel that she had the right to be given credit for the original photographs. Without Susan being present at that moment, capturing that Nicaraguan man's emotion, Joy Garnett would never have created her own version of the Molotov Man. So without Susan, there would have been no catalytic element to Joy's art exhibit.
Joy had every right to take the photograph and recreate/appropriate Susan's photograph. Joy wasn't copying the photo and passing it off as her own. She was inspired by it and made an entirely new piece of art, using an entirely different medium from the original. What Joy did was not plagiarism or wrong.
Luckily this little situation seemed to just work itself out. I understand Susan's frustration that her image suddenly became this symbol for something entirely out of context from the original. But a little blurb in an art exhibit brochure would have taken care of that. I only think there is an issue with appropriating original work when the appropriator tries to take full credit for the concept/idea/medium/everything. And I do think that there are people out there who would, and probably do, do just that. Which is a shame, because why would you not want people to see and share the artwork (assuming it's art), and future works of art by the same artist, that inspired you to create something new, with your friends.
Wasn't there a similar problem with the red, white, and blue Obama faces? Where one person thought another person's style of artwork was awesome, so they mimicked it, maybe even copied it, onto the face of Obama. I'd be flattered if I were the original artist, people were wearing those shirts everywhere.
In the plain and simple, I don't think it's wrong or inappropriate, dishonest, plagiarism, illegal, to take any work by another person, recreate it and make it your own NEW and unique work. However, I do think it is fair to give credit to the original artist. I think it'd be neat to see something made by me, appropriated. It's cool how we all see certain things and think differently about them.
With the digital age making the world available to us, I think it's important that we try and retain our uniqueness, individuality, and find common ground within. I think appropriation of artwork is damn cool.
ps- The Molotov Man looks like Che Guevara, no?