Interesting. Only the .jpg version is coming through on my feed. If I go to my profile, both .heic and .jpg are there in My Media, but only the .jpg is viewable. The .heic has an "I can't show you this image" icon, instead of the thumbnail picture. When I click on it I can download the .heic file, but not view it. The download reconstitutes fine on my (Mac) computer. It is in some sort of limbo on this site. Just to be clear, my gripe is entirely with Apple, and I don't expect this site to accommodate their latest effort to keep me within the Apple universe.
Ok, I finally found out I can change my iPhone's photo format to just .jpg, (Settings/Photos/Format>Most Compatible). I will leave this up in case anyone else has problems with iPhone photos.