cage.sh

#!/bin/sh

# Supported desktop environments
# 1. Aqua (MacOS)
# 2. Gnome
# 3. Unity
# 4. Feh
# 5. xfce4
# 6. KDE

onError() {
    echo -e " \u2639 Cannot cagify $1 desktop environment"
    exit 1
}

if [ -z "$TMPDIR" ]; then
    TMPDIR="/tmp"
fi

# [ -d $TMPDIR ] || mkdir $TMPDIR

curl -s -o $TMPDIR/cage.jpg https://user-images.githubusercontent.com/2625584/35465045-a69a87e2-02f9-11e8-98a3-10770c6a339f.jpg

if [ -x "$(command -v osascript)" ]; then
    osascript -e "tell application \"Finder\" to set desktop picture to POSIX file \"$TMPDIR/cage.jpg\"" || onError "Aqua"
    exit 0
fi

if [ -x "$(command -v gsettings)" ]; then
    gsettings set org.gnome.desktop.background picture-uri file://$TMPDIR/cage.jpg || onError "Unity/GNOME"
    exit 0
fi

if [ -x "$(command -v feh)" ]; then
    feh -g 1920x1080 -d -S cage.jpg $TMPDIR || onError "Feh"
    exit 0
fi

if [ -x "$(command -v xfconf-query)" ]; then
    xfconf-query -c xfce4-desktop -p /backdrop/screen0/monitor0/workspace0/last-image -s $TMPDIR/cage.jpg || onError "xfce4"
    exit 0
fi

if [ -x "$(command -v dbus-send)"]; then
    dbus-send --session --dest=org.kde.plasmashell --type=method_call /PlasmaShell org.kde.PlasmaShell.evaluateScript 'string:
    var Desktops = desktops();
    for (i=0;i<Desktops.length;i++) {
        d = Desktops[i];
        d.wallpaperPlugin = "org.kde.image";
        d.currentConfigGroup = Array(
            "Wallpaper",
            "org.kde.image",
            "General");
        d.writeConfig("Image", '"file://$TMPDIR/cage.jpg"');
    }' || onError "KDE"
fi

echo -e " \u2639 Your desktop environment isn't supported"
exit 1

I added KDE support. (untested)