Some images not working

Certain images on my site aren’t working. They don’t appear in the media area (just grey box instead) and cannot be entered into the post.

However, when I try to edit them they appear. Once I’m done editing they disappear again.

This is the site: rghub.org.uk

This is an example of one of the images I want to insert: it’s part of a series of other social media icon, most of which uploaded fine.
http://rghub.org.uk/wp-content/uploads/2013/09/Google-plus-square-e1379085097153.png