I just discovered this problem myself. I bought a new iPhone yesterday, and now I can’t upload photos using the website any more, but at least I was able to upload a photo by routing it through the iNaturalist app, thanks to @nomolosx’s comment above.
Edit: I found this other topic https://forum.inaturalist.org/t/iphone-images-heic-are-no-longer-compatible-w-inat/24036, in which @earthknight explained how to adjust the settings on the iPhone. I can now upload photos directly onto the website without going through the iNaturalist app again.