1) The majority of desktop tags are used to reference images which can conceivably be used as a wallpaper for a desktop.
2) That generally just means images of a specific dimension, from a range of dimensions.
3) Which is quite vague in itself, since there are so many possible desktop screen resolutions/sizes.
4) I propose we wipe them all. Except for the rare few which actually shows a desktop or wallpaper. (And some future functionality to search for a specific image size range)
Updated by Anomynous