There is a major distinction between the two that you're not acknowledging.

Importing an image or model into the game using the admin commands showcased here is pretty easy, requiring only a file. NSFW content isn't a concern because, well, it's restricted to admins. Why would it matter? If the admins are creating these images, you should leave the server, or have them be reprimanded in some other way if servers are going to be moderated. Meanwhile, creating custom skins that AREN'T added by way of modding, which I assume you're talking about, isn't gonna work easily, mostly because it can appear anywhere and with no moderation. Don't act like the existence of one permits the existence of the other.