Here's what I use to tag jpg images with exiv2 and dmenu, using Exif.Photo.UserComment and Iptc.Application2.Keywords:

Install dmenu and exiv2.

The shell-script image-metadata.sh does the actual work, we tell feh to use it for two actions and the info:

• edit-comment as action1 for editing the comment
• edit-tags as action2 for editing the tags
• show for --info

I put the following into my .bashrc

alias feh="feh --action1 \;\"image-metadata.sh edit-comment %F\" --action2 \;\"image-metadata.sh edit-tags %F\" --info \"image-metadata.sh show %F\" "

The file image-metadata.sh is as follows:

#!/bin/bash
if [ $# -lt 2 ]
then
    echo -e usage: "$0 <action> <filename>\n actions: edit-comment, edit-tags"
    exit -1
fi

action=$1
file=$2

if [ "$action" == "edit-comment" ]
then
    commentText=$(echo | dmenu -t "$(exiv2 -Pt -g Exif.Photo.UserComment $file)")
    if [ $? -ne 1 ] # not aborted
    then
    if [ -z "$commentText" ] 
    then
        exiv2 -M"del Exif.Photo.UserComment" $file
    else
        exiv2 -M"set Exif.Photo.UserComment $commentText" $file
    fi
    fi
fi

if [ "$action" == "edit-tags" ]
then
    exiv2 -Pt -g Iptc.Application2.Keywords $file > /tmp/._image_keywords.txt

    selection=$(exiv2 -Pt -g Iptc.Application2.Keywords $file | dmenu -l 10)
    if [ -n "$selection" ] 
    then
    exiv2 -M "del Iptc.Application2.Keywords" $file
    while read keyword
    do
        if [ "$selection" != "$keyword" ]
        then
        exiv2 -M "add Iptc.Application2.Keywords String $keyword" $file
        else
        deleted=true
        fi
    done < /tmp/._image_keywords.txt

    if [ -z $deleted ]
    then
        exiv2 -M "add Iptc.Application2.Keywords String $selection" $file
    fi
    fi
    rm /tmp/._image_keywords.txt
fi
if [ "$action" == "show" ]
then
    comment=$(exiv2 -Pt -g Exif.Photo.UserComment $file)
    exiv2 -Pt -g Iptc.Application2.Keywords $file > /tmp/._image_keywords.txt
    echo -n Comment: $comment, "Keywords: "
    first=true
    while read keyword
    do
    if [ $first == "false" ]
    then
        echo -n ", "
    fi
    echo -n $keyword
    first="false"
    done < /tmp/._image_keywords.txt
    echo
    rm /tmp/._image_keywords.txt
fi