That would require a hell of a lot of time. If anyone's curious enough to want to check what all available maps look like in and out, they can download the map themselves. If you're so keen on fulfilling your idea, go ahead and take screenies of all 300 and some maps. :P
It could start from now on. So new maps would have this option avaiable and the old ones woudnt, but maybe with time ppl could slowly upload screenies of the old maps... The idea is nice.
As it is now, there is not enough interest in the older maps (i.e. downloads) to generate entire 800x600 screenshots for them. But even for the newer maps, the thumbnails tend to be scaled down versions of 800x600 or larger resolution shots. You could make a similar argument by saying that you need more than one screenshot to get a good feel for the map. If people are interested enough, I feel it would be easier for them to download and try out the maps.