A little bash script to make your images 16x9 via adding white border stripes.

fix_aspect.sh

#!/bin/bash

if [ -z "$1" ]; then
    echo Usage: ./fix_aspect.sh '<img_file>'
    echo
    echo This will create a new image file with the word 'fixed' appended before
    echo the filename extension. Eg, myimg.jpg will become myimg.fixed.jpg.
    exit
fi

resolution=$(identify $1 | awk '{print $3}')
orig_w=$(echo $resolution | cut -f1 -dx)
orig_h=$(echo $resolution | cut -f2 -dx)

new_h=$(python3 -c 'print('$orig_w' * 9 // 16)')
new_w=$(python3 -c 'print('$orig_h' * 16 // 9)')

output_stem=${1%%.*}
output_ext=${1##*.}
output_fname=${output_stem}.fixed.${output_ext}

if [ $new_h -lt $orig_h ]; then
    # Use new_w to increase the width.
    convert $1 -gravity center -extent ${new_w}x${orig_h} $output_fname
else
    # Use new_h to increase the height.
    convert $1 -gravity center -extent ${orig_w}x${new_h} $output_fname
fi

echo Fixed image saved to the file: $output_fname