This was a recurring theme in the previous wallbase site: there's no real hard and fast "rule" that says that a certain wallpaper is NSFW or not, it's up to judgement.
In my opinion, an NSFW wallpaper is one where, if you work in an office environment and your boss catches you with that on your screen, you will almost definitely get a firm reprimand from HR. Sketchy means it depends on the type of boss and office environment, but at the end of the day, it still depends on the office environment you work in. I've been in some really sketchy workplaces before and some very strict ones, so there's still no hard and fast rule.
You could say the appearance of genitals is an immediate NSFW, sure, but as in that Cortana wallpaper, there's still some form of confusion. You could say that ass that's not in a bikini is NSFW, and that in a bikini is considered "sketchy", but how small does a bikini have to be before you consider it NSFW?
It's a massive gray area that there's really no solution for unless you come up with a mathematical algorithm that takes everything into consideration (which I guarantee no one's going to bother putting time into making an advanced picture recognition algorithm just to determine what level of ass is allowed on your computer screen at work.
At the end of the day, ask yourself this: if HR came to your computer and saw your screen, what would they think?
