This seems to be a recent problem with the iPhone HEIC photo format which is not recognized by web servers.
The device should convert the photo to a widely compatible JPG, but not all photos are being converted.
Change your iPhone/pad settings to make it automatically convert photos to a more compatible format when transferring.
Follow the instructions below to do so:
Go to Settings > Photos.
Scroll down to the Transfer to Mac or PC section.
As with all photo problems, don't struggle you can always send them over by Email, WhatsApp or Facebook & they will be uploaded for you.