How to open iPhone HEIC photos anywhere
Convert them to JPG with HEIC to JPG, or change your iPhone's camera setting so new photos are JPG.
Which format to pick
JPG suits sharing and upload forms. PNG suits editing. If a form wants a document, HEIC to PDF may be the better route.
Stop it happening again
On your iPhone, open Settings, then Camera, then Formats, and choose Most Compatible. Photos you've already taken stay as HEIC.