“Appropriation in art is the use of pre-existing objects or images with little or no transformation applied to them. The use of appropriation has played a significant role in the history of the arts (literary, visual, musical and perfoming arts.
Appropriation can be understood as “the use of borrowed elements in the creation of a new work.” In the visual arts, to appropriate means to properly adopt, borrow, recycle or sample aspects (or the entire form) of human-made visual culture. Notable in this respect are the Readymades of Marcel Duchamp. Other strategies include “re-vision, re-evaluation, variation, version, interpretation, imitation, proximation, supplement, increment, improvisation, prequel… pastiche, paraphrase, parody, homage, mimicry, shan-zhai, echo, allusion, intertextuality and karaoke.” The term appropriation refers to the use of borrowed elements in the creation of a new work  (as in ‘the artist uses appropriation’) or refers to the new work itself (as in ‘this is a piece of appropriation art’).” http://en.wikipedia.org/wiki/Appropriation_%28art%29
Not much that can be done about this one. My artwork was appropriated by an anonymous graffiti artist in an activist group fighting an airport in Norte Dame des Landes in France. My image is the third one down on their web page. Appropriation in art is part of the process. Art is used for graffiti, protests, political cartoons. Not much that can be done about it. It is kind of cool though to see my artwork used in graffiti. However Artists must always be vigilant and must be enforcing their copyright. If an artist does not, the artwork could become public domain due to lack of enforcement. I may not be able to do much about this graffiti but if this group decided to publish this image or use it to profit from then I can enforce my copyright. Links to my artwork: