One thought...Apple moved to a new format .HEIC a while ago. I'm not 100% sure that's compatible on the forums. On our iPhones/iPad's I always go to Settings -> Camera -> Formats and change the setting to 'Most Compatible' so it takes photos & video's in JPEG/H.264 vs the oddball HEIC/HEVC format.
2nd...if you're attaching a photo from your phone/computer, you cannot use the 'Insert Image' button, that's for Image 'Links' where you're using a service like Photobucket to host the image. If its on your computer, you use the ' + Attachments and other options' link directly below and then browse and attach the photo.