arch/pkg/de-p1st-xfce4/skel/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-desktop.xml

<?xml version="1.0" encoding="UTF-8"?>

<channel name="xfce4-desktop" version="1.0">

    <property name="desktop-icons" type="empty">
        <!-- Show desktop icons on primary desktop,
        otherwise some of them might be invisible (as they are too far down the screen)
        when using HiDPI -->
        <property name="primary" type="bool" value="true"/>
    </property>

    <property name="backdrop" type="empty">
        <property name="screen0" type="empty">
            <property name="monitor0" type="empty">
                <!-- Change background -->
                <property name="image-path" type="string" value="/usr/share/backgrounds/de-p1st.jpg"/>
                <property name="image-show" type="bool" value="true"/>
                <property name="image-style" type="int" value="0"/>
            </property>
        </property>
    </property>

</channel>