Jhead

From Wiki

jhead (see http://www.sentex.net/~mwandel/jhead/)

it allows to read the exif information of a .jpg file

It allows also to add a comment in the exif information (I'm not sure about that ... it seems that the exif comment is not standardized)

Anywway, ... we can use it. It is much more easy to store the assiciated info in th efile itself.