If you've ever tried to upload an iPhone photo to a website and gotten a cryptic error, or found it wouldn't open on a Windows PC without extra software, the culprit is almost always the file format: modern iPhones save photos as HEIC by default, not the JPG that most non-Apple software expects.
Why Apple switched to HEIC in the first place
HEIC uses a more modern and efficient compression method than JPG, storing visually comparable image quality in roughly half the file size. For a phone where storage is limited and users take an enormous number of photos, that space saving is substantial over time, which is exactly why Apple made it the default starting with iOS 11 rather than an opt-in setting. HEIC also supports some technical advantages JPG lacks, like storing image bursts and depth data (used for portrait-mode background blur) inside a single file.
Why so many apps and websites reject it
HEIC is a relatively newer format, and JPG has a multi-decade head start as the universal standard baked into essentially every image tool, website upload form, and operating system in existence. Many web forms, older Android devices, and Windows systems (without the extra HEIC extension installed) simply haven't added support, so from their side the file just looks unreadable. This isn't a sign anything is wrong with the photo itself, just a compatibility gap that's gradually closing as HEIC support becomes more widespread.
Converting HEIC reliably
The most dependable fix is converting HEIC to JPG before uploading or sharing, which HEIC to JPG does directly, preserving the image content in the universally supported format. If you have a whole camera roll's worth of HEIC photos to convert at once (for example, before importing into an older editing program), Batch Image Converter handles the conversion across many files in one pass instead of one at a time. You can also change your iPhone's camera settings (Settings > Camera > Formats) to shoot in "Most Compatible" mode going forward, which saves new photos as JPG automatically, though existing HEIC photos already on your device still need converting.