Thanks for the answer. You weren't late at all.
It is a mystery that you couldn't drag&drop the pictures nor could you right-click them, as I was still able to do so on my Mac... However, I am pleased to inform you that I tried the solution you let me know and the problem is now solved!!!
Now I can neither drag&drop nor right-click pictures any more, which is exactly what I have wanted.
I didn't expect that all slideshows were controlled by only one html file located in a folder apart from the gallery folder, though. It seems quite tricky to me
Thanks again for your help.
I can have good sleep tonight.