Pay close attention to the images in two vertical columns shown in this wallpaper. The left column shows the U.S.S. Enterprise NCC-1701 as it appeared in the 1st pilot episode of the Original "Star Trek" series (titled "The Cage") and the right column shows the same ship as it appeared in the same series' 2nd pilot episode (titled "Where No Man Has Gone Before"). Basically, you're seeing the first two of the three variants of the NCC-1701 that were shown throughout the series itself.

Enjoy, "Star Trek" fans!