Why is my phone saying this?

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This is a setting is iOS 11 and later that changes the image format to store more information in less space. Think of it as jpg+. Standard pictures are taken in jpeg. Apple has a new compression method that allows the same quality in less space. However, it is possible that this new format could be causing your problems for your outgoing pictures. We can set it back by going into Settings/Camera/Formats. Then choose most compatible.

https://support.apple.com/en-us/HT207022