You will want to use mogrify, but overlay (actually underlay) the background using a -draw image command.
See the example useing mogrify to frame images in IM Exmaples
http://www.cit.gu.edu.au/~anthony/graph ... s/#mogrify
It is basically exactly what you are wanting.
WARNING: make a backup of images before using mogrify. It is dangerious!
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