When taking photos with newer iPhones (iOS 11+) and importing to a Mac, the default image type will no longer be JPG. This new format that the Mac operating system uses is known as HEIF (High Efficiency Image Format).
Click here for an article that explains this new file format, as well as instructions for changing the iPhone settings so all future photos will be saved as JPGs.
Matrix will not accept these photos since they do not have the .jpg extension (their extension is .heic). This error will come up when you select an HEIF/HEIC photo:
These photos must be converted to JPG images in order to be uploaded.
Here is a website that allows you to convert HEIC photos to JPG for free.