Apple uses High Efficiency Image File Format for Photo’s and this in photo’s with the .HEIC extension. you can easily convert this on Ubuntu using;

sudo apt-get install libheif-examples

for photo in *.HEIC; do heif-convert $photo ${photo/%.HEIC/.jpg}; done

Alternatively you could disable the HEIF format on your iPhone under Settings > Camera; https://support.apple.com/en-us/HT207022