You should only post photos taken by others if you saw the specimen, people posting others’ photos just because people sent them to them, they’re not doing the right thing.
It’s not hard to create a separate profile for other collectors that send you their findings, you can then send them the email and passwords in the case they want to check/use iNat by themselves later.
I’m pretty sure it was discussed in https://forum.inaturalist.org/t/museum-specimens-antique-photos-and-inaturalist/31638