Apple's iPhone photos are currently in the HEIC (high-efficiency image coding) format, an unusual file type that not every app/software support at the moment. If you do not want to use this standard, such as when editing images, you can change the format in Camera settings. Just open "Settings", find "Camera", and then tap "formats". Finally, select "most compatible". This will also allow your videos to come out as H.264 instead of HEVC.