Someone know a way to send on Linux an "instruction" (like a system command) to an already open feh instance to reload the same image already loaded or maybe another one? I have seen feh doing relatively complex task via shell scripts, but I am struggling to obtain just this feature. Thanks in advance!
You can pass the -R flag to get it to reload every few seconds.
It's also possible that sending SIGUSR1 (if you know the PID, that's as easy as kill SIGUSR1 $pid) would be sufficient to get it to reload; that usually forces it to go to the next photo, but if there is no next photo it might force a reload of the current one.