It seems that the bug may be in the xfsettingsd process, which eventually stops copying images for reasons that are unclear.
workarounds might involve using clipman or killing & restarting xfsettingsd. It might also be fixed upstream in a new version, which also consolidates the clipboard code into libxfce4ui, shared between clipman & xfsettingsd. https://bbs.archlinux.org/viewtopic.php?id=294394&p=2 https://gitlab.xfce.org/xfce/libxfce4ui/-/merge_requests/117
