This took half an afternoon to sort out, so is probably worth passing on: automatic display of image thumbnails in one's file manager can be a security issue, so for some security-conscious distributions (QubesOS, Whonix, probably Tails) it is turned off by default. If one wants to turn it on for Nautilus in one Debian-based QubesOS virtual machine (for instance) run this at startup:
gsettings set org.gnome.nautilus.preferences show-image-thumbnails always
Alternatively, if yours in turned on and you wish it off:
gsettings set org.gnome.nautilus.preferences show-image-thumbnails never
should get the job done. To get the current setting:
gsettings get org.gnome.nautilus.preferences show-image-thumbnails
To get your options:
gsettings range org.gnome.nautilus.preferences show-image-thumbnails