How to forbide access to a folder and the pictures inside?

On my website, I have a folder which contains many pics. Some of my .php files with html display them with other content. The problem is that if you have the url to the picture or the folder that contains them, you can have an access to all of the pictures or the picture itself whenever. My question is How can I forbide the access to the folder and the pics except if the .php files want to display them ?. Thank you !