If you have an image in Drive that Google doesn't "see" when you try to Insert, or if you tried to copy/paste an image from your phone (this method is unreliable and not recommended) and it doesn't print, the solution is below.
The issue is that newer iPhones are set to save HEIC (compressed) images but Google has not yet implemented support for these so gDocs doesn't "see" them. iOS will now convert HEIC images when you email or text them, or you can upload photos to https://heictojpg.com/, download as JPG and insert.
The long-term option if you have enough space on your phone is to change the setting in Settings --> Camera --> Formats to "Most Compatible" and your phone will save "normal" JPG images and MP4 videos. True: this will use a bit more space on your phone but you won't have trouble using your media in other tools. You can also just change this setting when you know you are taking a picture that you want to insert into a document or slideshow.
