Overview
By default, iPhones save photos in the HEIC format, which may not be compatible with all devices or websites. You can convert HEIC photos to JPG or PNG when exporting, or adjust your camera settings so that future photos save in a more compatible format.
Here's How
Exporting an individual photo as JPG or PNG
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Open the Photos app on your iPhone.
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Select the photo you want to share or export.
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Tap the Share (export) icon. 
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At the top of the screen, tap Options.
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Under Export As, choose Most Compatible.
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Complete your export or share. The photo will be saved in JPG format instead of HEIC.
Changing your default camera format
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Open the Settings app.
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Scroll down and tap Camera.
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Tap Formats.
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Choose Most Compatible.
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New photos and videos will save as JPG, PNG, JPEG, or H.264.
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To save space using HEIF or HEVC formats, switch back to High Efficiency.
Additional Resources
For more information, head to the latest Apple Support article discussing this: Apple Support: Using HEIF or HEVC media on Apple devices.