I think that the title is a bit misleading. Indeed, this image is the most detailed infrared image of the galaxy[1]. Also, it is the most detailed image of the galaxy centre, since Spitzer can see through the huge amounts of dust that obscure the centre of of the galaxy in the optical wavelengths.

Caveat to my first point: with the advent of Herschel (which also performs in the near and far infrared wavelengths), more detailed pictures of the Milky Way emerged[2], although they have yet to image (or maybe just release) a full mosaic.

[1]http://apod.nasa.gov/apod/ap080605.html [2]http://herschel.esac.esa.int/FirstParallelModeImages.shtml