You move your iPhone photos to your computer and they won't open, or Word and online forms reject them. The culprit is almost always the HEIC format.
What HEIC is and why the iPhone uses it
HEIC (or HEIF) is the format Apple has used to save photos since iOS 11. It takes up about half the space of a JPG at similar quality, so more photos fit on your phone. The downside is compatibility: Windows needs an extension, many websites don't accept it, and several editing programs don't recognize it.
Three ways to solve it
1. Convert the photos. With HEIC to JPG, you drag them into your browser and download regular JPGs. It works on any system and your photos aren't uploaded to any server, which matters if they're documents, IDs or family photos. If you need quality with no extra compression, use HEIC to PNG.
2. Change the iPhone setting. Go to Settings › Camera › Formats and choose Most Compatible. New photos will be saved as JPG. Older ones will stay HEIC.
3. Convert when you send them. In Photos, when you share, you can choose "Options › Most Compatible" and the photo goes out as a JPG.
After converting: shrink the file size
A 12-megapixel JPG can weigh over 3 MB. If you're going to email it or upload it to a form with a size limit, run it through Compress images: at 75% quality and 1920 px wide, it almost always drops below 500 KB.
Quick questions
Do I lose quality? Going from HEIC to JPG at high quality is barely noticeable. Going to PNG loses nothing extra, but the file is much larger.
What about Live Photos or burst shots? The main image is converted. The video from a Live Photo isn't included.