In the transition to supporting multiple window managers with the ultimate goal of auto detection upon boot, in Tiny Core v2.2 I, introduced .setbackgrond, s a window manager agnostic tool.
With the introduction of this, it means that the local window manager's background capabilitiy should be avoided. This means {wm}_setbackgroiund script is depreciated. It should not be used.
What appears to be in your case, that you have a conflict of two systems tyring to set the background, the agnostic .setbackground and the jwm specific one via the <Backgroind ...> tag in .jwmrc Which is set via jwm_setbackground.
To solve comment out or remove the jwm <Background tag> in .jwmrc and avoid jwm_setbackground.
Then using Esetroot via .setbackgroind, the control panel GUI Backgrounds, or the wallpapger GUI will work for jwm-snapshot or with any window manager.