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.equals( image, true ) )
  						{
  							image.index = i;
  							return image;
  						}
  					}
  				}
  				else
  				{
  					for ( i = this.index + 1 ; i < snapshots.length ; i++ )
  					{
  						image = snapshots[ i ];
  						if ( !currentImage.equals( image, true ) )
  						{
  							image.index = i;
  							return image;
  						}
  					}
  				}
  			}
  
  			return null;
  		}
docs: Get the closest available image.