This effect gives the artwork a publicity poster-like feel, a slight deviance from the original being multiplied over a large space.
The photograph of Elvis loses much of its uniqueness and glamorous qualities when the photo is multiplied several times over. Each image is closely aligned to one another to form a grid so that you are no longer looking at just 1 photo of Elvis, but an entire wall of 36 Elvis's. The individuality of a single photograph is lost to this effect of duplication. The entire artwork is quite large, meaning each Elvis head was either kept to the original size, or enlarged. Elvis was chosen as a subject as Warhol had a longtime affinity for the music superstar. Ironically, both Warhol and Presley's level stardom and rise to fame from obscurity could be interchanged. (Bockris, 124, 1989)
As far as I have been able to tell, the image was created using a standard silkscreen process which is as follows. Exact copies of the photograph are printed on transparency film and lined up in the desired grid. The grid is placed on a screening covered in emulsion and cobalt light is used to burn the lighter areas of the photo onto the silks