Imagemagick

Convert PDF to tiff

convert foo.pdf pages-%03d.tiff

Convert Image to grayscale

convert <inputfile> -set colorpsaace Gray -separate -average <outputfile>

Overlay Image with text

Source

ww=`convert zelda3.jpg -format "%w" info:`
convert zelda3.png \( -background none -size ${ww}x -fill black \
-font Helvetica label:"restricted" \) \
-gravity north -compose over -composite zelda3_tmp.jpg

or

convert zelda3.png \( -background none -size $(convert zelda3.png -format "%w" info:)x -fill black \
-font Helvetica label:"restricted" \) \
-gravity north -compose over -composite zelda3_tmp2.jpg

Convert multiple images

Source

# Change size
mogrify -scale 800 -path /tmp/output img_*