Securing Uploads Directories underneath wp-content

I’m not sure if it’s due to the plug-ins that I have running or if this is a problem with core WP…..

I’ve found that when I post images on websites that are linked to images in a specific folder in my blog that the whole index is able to be viewed! as well as back-tracked to core wp-content folder which mercifully can’t be viewed. (hope that makes sense).

How do I protect this folder and the other folders that are also currently exposed?

Help please and many thanks in advance. 🙂