Advice worked like a charm - with ONE reservation. The choice a User can make is for ONE picture, whereas the System places FOUR by default on the folder.
You have thrown yet another light on this problem. It may be relevant to the issue I have outlined above to Tufenuf.
When I said folder 'icon' in my original post, I did not realise that the four pics that the system selects, show not on an icon, but on a TILE (selectable in View Menu).