You need to go to your steam / game instalation folder.
for me its D:\Program Files (x86)\Steam\steamapps\common\Deep Space Waifu
then you need to go to \Deep Space Waifu_Data\GirlsPhotos
There you can find one or two folders one called Wallpapers and the other one called Wallpapers_Academy

There you go here are all the wallpaper for Deep Space Waifu.