One way is ImageMagick - packages available for most linux/unix, cygwin on Windows, Homebrew on Mac. Using the convert utility from ImageMagick:
convert -quality 70 latest_hd.jpg smaller.jpg
latest_hd.jpg was 281 KB, 'smaller.jpg' file is 125 KB with not much noticeable loss of quality and same resolution.
you could use the program curl, also widely available on same platforms, to retrieve the image from the camera, run the convert tool, then use curl to upload to WU. All scriptable to run at set interval.