welcome to the forum :)
The taxon photos can be chosen/changed by any user. However, given these photos are meant to be a good representation of that entire taxon, and that many people will use these photos to compare to their own and help make identifications, there are a set of criteria that these photos should fufill. These include things like being high resolution, showing the entire organism in shot, depicting multiple important diagnostic characters, etc. For your photos to be displayed on the taxon page of a species, they should fulfill these criteria.
You can read some good discussions about this topic at https://forum.inaturalist.org/t/reminder-taxon-photos-should-include-multiple-life-stages/23852 and https://forum.inaturalist.org/t/what-makes-a-good-animal-taxon-photo/6739.
A good way to get your photos onto these pages is to observe rare species! Common species have thousands and thousands of photos to select from, so unless yours are extremely high quality, they’re unlikely to be chosen. Observing rare species with very few (or zero for unobserved taxa) photos will increase your chances.