Backgrounds in XFCE get set to their store path that over time get gc'd killing the background. Perhaps somehow making them point to /run/current-system/profile/share/backgrounds/xfce somehow would be better.
/home/b/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-desktop.xml 22: <property name="last-image" type="string" value="/gnu/store/fmds2rcqqxmiq1ipk6wqbf8x68ks93pp-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/> 27: <property name="last-image" type="string" value="/gnu/store/sc0lz41k6l41yyyrgh8k3x9cyvxwn418-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/> 32: <property name="last-image" type="string" value="/gnu/store/gym3674d636cxlk1jxgwqn3qv55hm99d-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/> 37: <property name="last-image" type="string" value="/gnu/store/gym3674d636cxlk1jxgwqn3qv55hm99d-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/> 60: <property name="last-image" type="string" value="/gnu/store/fmds2rcqqxmiq1ipk6wqbf8x68ks93pp-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/> 65: <property name="last-image" type="string" value="/gnu/store/sc0lz41k6l41yyyrgh8k3x9cyvxwn418-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/> 70: <property name="last-image" type="string" value="/gnu/store/gym3674d636cxlk1jxgwqn3qv55hm99d-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/> 75: <property name="last-image" type="string" value="/gnu/store/gym3674d636cxlk1jxgwqn3qv55hm99d-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/> 80: <property name="last-image" type="string" value="/gnu/store/l8psafxs8fhniyi0y6qycjv25bxm7pq6-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/>
