image_path='~/image.png'
message_line_1='Stop'
message_line_2='Not For Sale'

# Get picture height
height="$(identify -ping -format '%h' ${image_path})"
# Calculate preferable text point size based on picture height 
preferablePointSize=$((height/11))
margin1=$((-1*preferablePointSize))
margin2=$((preferablePointSize/2))
directory="$(dirname "${image_path}")"
filename=$(basename -- "${image_path}")
extension="${filename##*.}"
filename="${filename%.*}"

output=${directory}/${filename}_stamped.${extension}

convert -density 150 \
        -fill "rgba(255,0,0,0.25)" \
        -gravity Center -pointsize "${preferablePointSize}" \
        -draw "rotate -45 text 0,${margin1} \"${message_line_1}\" text 0,${margin2} \"${message_line_2}\"" \
        "${image_path}" "${output}"

echo "Result saved to: ${output}"