code: function( isUndo ) {
  			var snapshots = this.snapshots,
  				currentImage = this.currentImage,
  				image, i;
  
  			if ( currentImage ) {
  				if ( isUndo ) {
  					for ( i = this.index - 1; i >= 0; i-- ) {
  						image = snapshots[ i ];
  						if ( !currentImage.equalsContent( image ) ) {
  							image.index = i;
  							return image;
  						}
  					}
  				} else {
  					for ( i = this.index + 1; i < snapshots.length; i++ ) {
  						image = snapshots[ i ];
  						if ( !currentImage.equalsContent( image ) ) {
  							image.index = i;
  							return image;
  						}
  					}
  				}
  			}
  
  			return null;
  		}
docs: Gets the closest available image.
  
  @param {Boolean} isUndo If `true`, it will return the previous image.
  @returns {CKEDITOR.plugins.undo.Image} Next image or `null`.