Why iPhone photos will not open
Since iOS 11, iPhones have saved photos as HEIC rather than JPG. It is a genuinely better format, storing the same picture in roughly half the space. The problem is everything else: older versions of Windows, many websites, most upload forms and a great deal of software simply do not recognise it.
So a photo that looks fine on the phone that took it arrives as an unopenable file for the person receiving it. Converting to JPG solves that permanently, at the cost of a somewhat larger file.
Stopping it happening again
On your iPhone, open Settings, then Camera, then Formats, and choose Most Compatible. New photos will be saved as JPG from then on. Existing photos stay as they are, which is what this tool is for.
There is also a setting under Settings, Photos, at the bottom: Transfer to Mac or PC. Setting it to Automatic converts photos as they are copied off the phone.
Why this one does not upload your photos
Camera-roll photos are among the most personal files people have, and most HEIC converters ask you to send them to a company's servers. This one decodes them inside your browser using libheif compiled to run locally, so nothing is transmitted and there is no limit on how many you convert.
The trade-off is a 3MB download the first time, since the decoder has to reach your device. It is cached afterwards.
Questions
Will I lose quality?
At quality 90 the difference is not visible in normal viewing. If you want no loss at all, choose PNG, though the files will be several times larger than the HEIC original.
Does it keep the date and location?
No. Converting drops the embedded metadata, including GPS coordinates and timestamps. That is usually an advantage when sending photos to strangers, but worth knowing if you rely on photo dates.
Can I convert hundreds at once?
There is no limit imposed here, but each photo is decoded by your device, so a very large batch takes time and a phone may run out of memory. A few dozen at a time is comfortable on a laptop.