The "Options -Indexes" directive stops Apache from showing the default view of the folder's contents when it does not contain an "index" page. eg. where an HTTP request for www.yoursite.com/img/" would show something like
This does not stop someone from accessing the folder's contents once they know the details. eg. I could hotlink to your image using
and your image would show in my page.
<img src="http://www.yoursite.com/img/imgA.jpg" />
If you don't want anyone to be able to do that, you can put your image folder outside of your site's root folder (i.e. next to your "public html" folder) and access from your pages by "backing up" to it. eg. in your main index page located at www.yoursite.com/index.html you would use
<img src="../img/imgA.jpg" />