How to protect thumbnail and library images from media browser from being deleted?
You could try this module:

Soft delete


  This module allows selected users, based on a permission, to soft
  delete nodes. To users the operation looks like delete but the node is
  unpublished. It's useful if you want to prevent accidental deletes or
  bring back content that wasn't supposed to be deleted.
  
  Note that this is a very simple module, adding just a tab called
  Delete next to View and Edit on nodes that the user can unpublish. A
  more advanced and customizable alternative is the Content
  unpublish module.