The way the plugin works is that it grabs all images on your site that are attached to a page or post and display them. The reason the image needs to be attached to a post is so the wall can figure out where the image link should point to.

What the plugin does not do is look at the content of the post to see if you actually display the photo on the post. That would take much much much more processing power and slow down your server.

If you are not using the photo, then simply delete it from your media library or detach it from the page/post. Then it will not show up on the image wall anymore.