def thumbnail_dimensions(geometry)
      dimensions = ThumbnailDimensions.new(geometry, image.width, image.height)
      { width: dimensions.width, height: dimensions.height }
    end
def thumbnail_dimensions(geometry)
      dimensions = ThumbnailDimensions.new(geometry, image.width, image.height)
      { width: dimensions.width, height: dimensions.height }
    end
Intelligently works out dimensions for a thumbnail of this image based on the Dragonfly geometry string.