Right now the majority of beatmaps take the background in the .osu files to use as the background overlay/web thumbnail. I apologize if this has already been requested but, as a background is specify-able, why not thumbnails, for those storyboarded maps that just have to use a black BG by default (for dramatic effect!) but should still have an attention-grabbing thumbnail (as LH has done with maps like the SMB Medley, Tribute etc).