code: function setInvisible(el) {
  	    if (!el.holderData.invisibleId) {
  	        App.vars.invisibleId += 1;
  	        App.vars.invisibleImages['i' + App.vars.invisibleId] = el;
  	        el.holderData.invisibleId = App.vars.invisibleId;
  	    }
  	}
docs: Sets a unique ID for an image detected to be invisible and adds it to the map of invisible images checked by visibilityCheck
  
  @private
  @param el Invisible DOM element