code: function upToDate (imPath, type) {
            return readFile(imPath)
              .then(getChecksum)
              .then(function (sum) {
                var key = imPath + type
                var isUpToDate = cachedState[key] === sum
                cachedState[key] = sum
                return isUpToDate
              })
          }
docs: log.info('read image cache', cachedState)