I want to use the module Picture (https://www.drupal.org/project/picture) for the Node Gallery thumbnails.

Unfortunately the formatter Picture is not available in the (Gallery Item) Content: Gallery Media field in Image Thumbnail Grid.

Is there a way to use own Image field? I found this : https://www.drupal.org/node/1972212 but still do not understand how to do this.

Any help will be much appreciated.