Creativity often involves bringing together disparate ideas or images, so that the juxtaposition between them stimulates the viewer's imagination. Sometimes we want to give the viewer a great deal of detail, and sometimes we want to create a lot of 'space' for the imagination to run free. Artists have always sketched ideas, and 'sketching' with ready-made images in the form of papier collé
or collage is a valid way to both explore ideas and create finished works. Artists have always 'borrowed' images from each other, and collage is a well established craft and art technique, but the pop culture trends of 'remix' and 'sampling', following hard on the heels of postmodernism have seen it explode in popularity.
For the contemporary artist, internet publishing and the awareness of copyright can be a limiting factor; being inspired by a pop culture image can easily land the artist in legal and ethical hot water. So before you create any collage works, be sure to educate yourself about copyright, and find suitable creative commons images to source for your art.