Just put a file inside the images folder called index.html. It can be blank, or can have a message such as “Access Denied”. But with that file in there, it’s not possible to view the contents of the folder in the way you described.
Ralph, thanks for the quick answer. I have many subfolders in the images folder and it seems that I need to put an empty index.html file in every subfolder. Isn’t there a better approach?
yes. Also, if it is nothing to do with linux you can try to include .htaccess files in Apache and Windows. Come here and tell me the results of yout attempt.