That would by my suggestion. Something along the lines of “Photo taken by NAME, used with permission” or “Photo copyright exclusive to NAME, used with permission” would generally satisfy the legal side of things. Maybe an addition along the lines of “YOURNAME claims to hold no additional copyright” as a disclaimer as the name appearing on the file’s license is inaccurate.
Generally speaking, use with permission is a legal exception to copyright that doesn’t require meeting any requirements other than permission and statement of permission being given. (Other exceptions have inherent incompatibilities with iNaturalist.)