i was taught in art class to use reference sources if a model was not available & have scads of files of ripped out magazine pics... mostly from Nat'l Geo., if i want to get, let's say, an elephant anatomically correct... or a piece of architecture... this i consider ok... when you get into stuff that's recognizably someone else's work (or a portion of) then i think you could run into problems personally, i would be flattered, but it would depend on which image you wanted too :]