Reading about appropriation and the term, it is not clear on somethings if it is okay to do or not. "As a term in art history and criticism refers to the more or less direct taking over into a work of art of a real object or even an existing work of art." Even this term is not completely black or white.
There are a few things I think that are big factors to whether or not appropriation is legal. If an object that they are using is not created or made to be a piece of art, using it in your own art, I don't think should be illegal. Like Jeff Koons' Three Ball Total Equilibrium Tank, those basketballs were not created to be art, they were made for commercial reasons and it is fine if someone wants to use them as art.
Another big factor if what is being remixed was originally intended for art, is how recognizable it is in the new piece. Taking little pieces of it and distorting it should be fine. If there is one piece that is just taken from the whole image, and placed in the new image, then that should not be legal.
The last big factor, depends on the original artist. If they could care less about people using their art for other pieces, that is fine. If an artist does not want his or her piece to be remixed, people should respect that and not remix their work. It should not be up to the person who owns the piece, if a museum has a painting by someone that doesn't mind their work being remixed, the museum should have no say in if it is ok or not.
The majority of my images I would say are illegally remixed. The original piece that I took these from was intended to be a fine art painting. I doubt that the artist would be ok with me taking those images and pieces from it and using it for another image without giving him credit.