@foone hunh, PNG spec (circa 1996!) actually includes a way to do *content warnings* inside the PNG: https://www.w3.org/TR/PNG-Chunks.html#C.tEXt (not that I expect any viewer ever did a damn thing with it)

"Description" is arguably the right thing, and is already standardized! The only sticking point is that, as a thing around since 1996 with no(?) usage, it might be ruined by junk values in the wild.

Also I bet pngcrush throws these out in common usage.

sure sex is great but have you ever turned off an IBM PC AT by throwing the big red switch and been absolutely, supremely confident that all computing has now ceased and will not resume just because the operating system gets peckish for some midnight updates