docs: def get_image(self, digest, blob, mime_type, index, size=500):
          """Return an image for the given content, only if it already exists in
          the image cache."""
          # Special case, for now (XXX).
          if mime_type.startswith("image/"):
              return ""
  
          cache_key = f"img:{index}:{size}:{digest}"
          return self.cache.get(cache_key)