..Peter Scheldahl (who the fuck knows how to spell his name, not even him!...) wrote in some review a while back that "collage is the true language of Modernism", and he was right.
The notion that you can make a new picture but combining other pre-existing pictures (and in that process redefine both the original found source material and the resulting collage, which is always more than the sum of its parts...) is the basic strategy from which modern image-making, and thus contemporary art, proceeds. Collage is both a "media" for image-making and a general aesthetic. The idea of collage (the combining of known forms to make a greater/different whole, the shift in context when disparate parts are placed together within the same frame...) is present in every 20th century art form: architecture (the jagged teeth of an urban skyline), literature (multiple narrators telling different versions of the same event), even dance (fluid and broken movement in the same piece performed by the same body...)
The collage aesthetic creates both a quality of familiarity (with the found "pieces" that are slammed together) and estrangement (fracture, this slamming itself is often jarring and unsettling). It is this very contradiction that makes collage so popular and ubiquitous in contemporary culture.