my take is that the main photo should be whatever is the most charismatic thing. for example, a lot of oak galls are really interesting and beautiful (ex. Andricus coronus), but the wasps that cause them are somewhat nondescript. so the gall should be featured in the main photo. on the other hand, some, say, leaf mining flies are colorful and interesting (ex. Liriomyza commelinae), while their mines arre rather nondescript. so even if the adults are less often noticed by humans, i think a case can be made that the adult fly is more interesting than the mine. so the adult fly should get the main photo, if a good photo exists.
(similarly, i’m much more likely to encounter and make an observation of scat, tracks, and other coyote signs than an actual live coyote. but i don’t think there’s a question that a photo of an actual coyote deserves to be the default photo because the actual animal is more charismatic than its scat.)