Having screenshots is important, a lot of people pick and choose maps to download based on screenshots alone, so if you have none your map will not be appealing to anyone looking for new maps to check out.
You don't need pictures of every part of the map, just a decent small number of shots that show off what your map has to offer. It's also highly suggested that you take screenshots in spectator mode and disable the hud using cl_drawhud 0 to remove unnecessary visual noise.