Inappropriate Use of Digital Media

I teach 4th Grade and we have 1 to 1 Chrome books. I wish we would have had a way to block the students from changing backgrounds.  Multiple times last year I had students putting inappropriate pictures as backgrounds.  They didn't understand why Fortnight pictures with guns were not okay for school when they are allowed to play it and use the pictures at home.  I couldn't believe some of the pictures that they found and tried to use.  

This is large problem in high school as well.  In addition to violence there are always images that are sexually suggestive.