this request was sort of covered in a previous discussion: https://forum.inaturalist.org/t/data-users-what-are-your-use-cases-and-requests-for-exporting-data/2972/7.
currently, it is possible to get all image files for a set of observations (including multi-photo observations), but you have to do it via one of the get observations endpoints in the iNaturalist API (see https://api.inaturalist.org/v1/docs/#/Observations).
just curious – what is your use case for getting all photo URLs into a list? (if i understand what you’re trying to do, i might be able to provide more info on how to achieve what you’re trying to do.)