mkimg

#!/usr/bin/env bash
if [[ $# -eq 0 ]] ; then
    echo 'You must provide a size'
    exit 0
fi

# first make the image with no content
convert -size $1 xc:hotpink $1.png

# now add in the size
convert -size ${width}x30 -background '#00000080' -fill white label:$1 miff:- |\
  composite -gravity Center -geometry +0+3 - $1.png $1.png

e.g. mkimg 100x100 or mkimg 800x600

There's probably a way to do this in one command rather than two passes. Feedback welcome.