Split in two parts, chop and join a transparent button image with ImageMagick.

gistfile1.sh

#!/bin/sh
function split { # file, target_size
  _w=$(identify -format "%w" $1)
  _dx=$(( ($_w - $2) / 2 ))
  convert $1 -crop 50%x100% +repage parts.png
  convert parts-0.png -gravity East -chop ${_dx}x0 left.png
  convert parts-1.png -gravity West -chop ${_dx}x0 right.png
  montage -background none left.png right.png -mode Concatenate -tile x1 out.png
  rm parts*.png left.png right.png
}

split $1 $2
echo "Done: out.png"