I made an observation for iNaturalist, and my friend took the picture. He told me it was fine to use his picture, and I want to give him credit for the photo. Is there an option for this?
There’s nothing official, most people put it in the description.
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.)
In the Observation Fields of the observation, if you type in photographer, there are a few options to choose from in the drop down list where you can add the name of the photographer.