I think another aspect that should be considered is where an image is used. I'm not entirely happy about displaying explicit images as tiny thumbnails with a red line running across them no matter where in the wiki they are posted. Someone clicking on a page named "BDSM" should expect to find something there that isn't NSFW, someone clicking on page that describes someting sex-related should expect explict content, and the tiny thumbnails/red line thing could be really an issue when it comes to artist's pages because there we have always tried to accomodate the wishes of the artists who allowed us to upload the images in the first place.

And certainly such an image might be NSFW, which could be a consideration for many users who are looking at the pages in a place where they need to avoid NSFW images.

Apropos, NSFW would include pictures that don't necessarily show naked bodies. Anything showing Nazi symbols (for example Hetalia fanart) would be NSFW in Germany because it's against the law.