Sarno The filenames are irrelevant as they're not displayed to user in the wallpaper selectors.
I think the current selection of wallpapers is more than enough. I would imagine 90-95% of users would change their wallpaper shortly after installing anyway. The selection that is provided is purely there to be the default/small selection.
My /usr/share/backgrounds folder is already nearly 100MB. It's already a bit big IMHO. I do think some of those may have come from Plasma though.