As well as the official advice from iNat help, there are several helpful forum threads with recommendations about how to select taxon photos:
https://forum.inaturalist.org/t/what-makes-a-good-animal-taxon-photo/6739
https://forum.inaturalist.org/t/reminder-taxon-photos-should-include-multiple-life-stages/23852
https://forum.inaturalist.org/t/taxon-page-photo-philosophy/56948/6