It's an illustration inspired by a fanfic called Impossibly Delightful Flesh. (As the title suggests, NSFW). The illustration is perfectly work safe, though. You can easily recognize Giles and Willow, but everyone else has swapped sexes due to some demonic spell thingy. Everyone is standing with their Season 5 SO, with Spike and Buffy appropriately maintaining some distance. See if you can figure out who's who.
I can't vouch for the quality of the story, not having read it yet, but I think this illustrates what fic is for. I honestly think this would have made a great episode concept, if time and resources had been available to pull it off.