Hey Reiner,
the images in the “gallery” post and project formats are sourced from the images attached to the post in question, and the order is determined by the date when the image was attached – i.e. in case you’ve uploaded images into the project content, they will appear in the same order as they were being uploaded; in case you wish to reorder the images, you’ll need to delete them from the “Media -> Library” admin menu and upload again to the project in question, following the order you need.
Sincerely,
Andrii / Satori Studio