https://discourse.gnome.org/t/survey-custom-file-thumbnail-usage/18241
Custom Icons in Nautilus is not allowed anymore since version 46. It works for directories from UI only. Thus it is required to set a custom icon through command-line:
$ gio set $HOME/AppImage/BLAH.AppImage metadata::custom-icon file://$HOME/Pictures/BLAH.png
delete:
$ gio set -t string BLAH.AppImage metadata::custom-icon-name -d