I have no authority there, but I think reasonable conventions are pretty much common sense that falls out from the fact that you can flag posts as 18+ and the way tag muffles work. To summarize:
- Flag a post as 18+ if (and only if) it meets the criteria from the community guidelines: if it's pornography or nudity, either photographic or drawn; or other material which depicts or describes sexual acts in explicit detail.
- Add content warnings for things that are likely to be undesirable for many or most people to see and aren't already covered by the 18+ requirements above. This covers things like graphic depictions of violence or media spoilers.
- Add tags for things that are undesirable for some people to see, like common phobias, particular fetishes, or common triggers like food and alcohol. This allows people who care about those things to muffle the tags—upgrading them to content warnings for themselves—without requiring extra clickthroughs for everyone else.
There's nothing intrinsically different between text posts and image posts here, except that text posts can mention body parts without actually depicting nudity, in which case they don't require an 18+ flag.

