ImageMagick Rules!
Part of my current project is to come up with a tool to layer various images on each other and finish with a composite picture. ImageMagick does this neatly and automatically. For example:
$ composite -verbose layer_foreground.png -compose Over layer_background.png final.png
We can put random text into an image:
$ convert -verbose -font $fontfile -fill grey -gravity North -draw ‘text 0,25 “text string”‘ layer_background.png
To make a small rectangle of color:
$ convert -size 20×10 xc:red smallred.gif
Many, many other things. The ImageMagick documentation is excellent.