Hi, I don't know if it's just me but I have noticed that people who are not registered can also see the wallpapers from the NSFW category.
When you open a NSFW wallpaper it says Wallpaper Blocked! but you can still see the ID in the link (for example "https://alpha.wallhaven.cc/wallpaper/**706095**").
All you have to do is open SFW wallpaper and then open the image in a new tab with right-click mouse button (link looks like this "https://wallpapers.wallhaven.cc/wallpapers/full/wallhaven-**706106**.jpg").
Then just change the ID in that link to the NSFW one and you can see the it.
You can see all wallpapers here by changing the ID number in that second link I posted.