According to Apple, they default to HEIC because it is the most compact, so I would say only do this if you have plenty available memory in your iohone, and you clear it down farly regularly. This link describes how to revert to JPG format.
https://forum.inaturalist.org/t/heic-files-not-being-permitted/68333
Sorry if link posted badly, learning this phone and forum myself !!
I have found it very easy to follow what another member suggested - create observation from first photo, save it and open observation. There is a small pencil icon by the photo, click that and either take additional photos (my m.o.), or upload additional photos from your phone cam gallery. I found uploading photos retrospectively a problem if you made a lot of observations because it was not so easy to match the photos to the observation (especially if it was photos of leaves !!).