Mocpages user, Mark B. , shows it's possible to construct a great looking mosaic without all the pieces in the world.
This small Darth Vader was made entirely out of pieces found at a Pick-a-Brick wall at a LEGO store. This helps though when you work at a Lego store, of which Mark does. How long it took and how he designed it is a mystery. ;-D
What's not a mystery is his admiration and obsession for The Nightmare Before Christmas movie by Tim Burton, as displayed by two of his creations. The first one is a humongous eight feet tall and four and a half feet wide mosaic of the characters.
Of which this mosaic certainly uses a greater variety in pieces than the Darth Vader one.
And second, we have the Night"mare" stand that features several mosaics and working parts.
If you'd like to see anymore of Mark's creations, check out his Flickr here, his Mocpage here, and the creations featured here, here, and here.
"And on a dark cold night, under full moonlight, he flies into the fog like a vulture in the sky! And they call him, Sandy... Clawssss...!" - Jack Skellington