An HEIC file (photo) means you are using an iPhone and taking photos using their new file format. However, that new format isn't supported as a standard on the web (yet). You'll need to save a copy of the photo in a format this website supports. Certainly JPG, probably GIF or the more robust PNG, and maybe a few others.
I hope that helps!
Steven
--- Minor update/edit: I did some quick research and learned this isn't just an Apple thing. However, I still stand behind the rest of my response!
sjt