If you post an image that is sized to a 16:9 ratio, it won't be cropped at all when the thumbnail is displayed. This is nice, especially for text so you can just read it inline.
Enter the Swiss Army knife, ImageMagick (
brew install imagemagick if you're using Homebrew):
convert source.png \ -gravity center \ -background white \ -extent '16:9#' \ dest.png
This will extend the canvas of
source.png with a white background until it is 16:9 and output the result as