Current icon for "select all" in configuration page looks like "image not found" icon.
See the screenshot attached.

Typical Image Not Found icon: http://th01.deviantart.net/fs71/200H/i/2012/324/c/0/image_not_found_icon_by_oldskull-d5llmx5.jpg

I would suggest to use an icon that clearly shows "select" like this:
http://icons.iconarchive.com/icons/kyo-tux/phuzion/256/Sign-Select-icon.png