What is censorship by omission?

Something that has been left out of has been neglected. When someone has been given a responsibility and has failed in that responsibility. It is neglecting to do something; leaving out or passing...

http://dictionary.law.com/Default.aspx?selected=1391

Censorship is the editing of material from a written or visual presentation. Sometimes the word "censored" is used to indicate that material was removed in the interest of morality, or would be...

Noun1. something that has been left out or passed over2. an act of missing out or failing to do something: we regret the omission of these and the names of the other fine artists

It is the banning of pictures that display arguable content.