You need to install the following:
  sudo apt-get install libimage-exiftool-perl

Create a bashscript:

<file>
#!/bin/bash
IMAGEDESCRIPTION=""
IMAGEDESCRIPTION=`zenity --width=500 --height=50 --title "Image Description" --text "Input text" --entry `

if [ -z $IMAGEDESCRIPTION ]; then 
        exit 0
   else
        FILES=`zenity --file-selection --multiple --separator=" "`
fi

exiftool -imagedescription=$IMAGEDESCRIPTION $FILES
</file>

This script will ask for a text string which then will be added to the selected files under the tag "Image Description".
