I use digiKam when editing my photos. Adjusting colors and balances and cropping. Then before outputting the end result, I resize. I guess it is actually 2 clicks, or 3 clicks extra.
672 x 391 is the perfect proportion for thumbnail images.
Also, since I use linux, I could just run it all from the command line:
ffmpeg -i input.jpg -vf resize 672x391 output.jpg
and you can tell it to resize the entire directory.