code: func cacheKey(bref string, width int, height int) string {
  	return fmt.Sprintf("scaled:%v:%dx%d:tv%v", bref, width, height, images.ThumbnailVersion())
  }
docs: // Key format: "scaled:" + bref + ":" + width "x" + height
  // where bref is the blobref of the unscaled image.