code: function(imgPath, callback) {
          this.getImageChecksum(imgPath, function(err, checksum) {
              if (err) {
                  return callback(err);
              }
  
              this.imageWithChecksumExists(checksum, callback);
          }.bind(this));
  
          return this;
      }
docs: Checks if a given image exists on the server
  
  @param {String}   imgPath
  @param {Function} callback
  @return {ImboClient}