Example:
convert -resize 800x600 -quality 80% -strip -interlace Plane -pointsize 150 -font Tahoma -draw "gravity center fill RGBA(200,200,200,0.3) text 0,0 'WATERMARK'" source.jpg output.jpg
Replace WATERMARK with your text
RGBA, A is alpha channel, a number between 0.0 (fully transparent) and 1.0 (fully opaque)
