What is HEIC?
HEIC (or HEIF) is the photo format iPhones use by default. It stores photos in roughly half the space of JPG, but many websites, forms and older programs do not accept it.
Fastest way: convert online
Open the HEIC to JPG converter, choose your photos and download the JPG files. It runs in your browser, so nothing is uploaded, and you can convert several photos at once.
Stop your iPhone saving HEIC
On an iPhone, open Settings, then Camera, then Formats, and choose "Most Compatible". New photos will be saved as JPG. Photos already taken stay HEIC.
Share as JPG from iPhone
When you send photos by email or upload them, iOS can convert them for you in some apps. Choosing "Most Compatible" or sending the original and converting on a computer gives you a JPG.
On Windows
Windows may need the HEIF Image Extensions add-on to open HEIC. If you only need a JPG, converting in the browser is simpler and needs no install.
If the JPG is too big
Reduce it with the compress tool, for example to 200KB or 1MB.