Summary:

When adding custom field images in the card layout using the Board Settings for Scrum or Kanban Board, the images/thumbnails are fetched from the issue attachment. However, there is no pattern in which the images/thumbnails are shown.
When there are multiple images attached and removed from an issue simultaneously, there is no clear pattern or rule on which image will be selected as the Card Thumbnail.

Would also like to know if there is a priority-based logic behind the Thumbnails on the Card of a board.

Steps to Reproduce:

1. Go to a Classic Project and create a Scrum or a Kanban Board.
2. Select ... 3 dots from the top right corner and click on "Board Settings".
3. Go to "Card Layout".
4. Click Add and then select images as the field name
5. Go "Back to board".
6. Open an Issue and add multiple attachments randomly using comments, Add attachment button, and, using the + button located above all attachments in the Issue View.
7. Delete few comments(with attachments), Delete attachments from the list in the random selection and there is no predictability with what could be the next image/thumbnail of the Card on the board.

Suggestion:

1. Make the most recent and latest .png or, .jpeg attachment in the list the thumbnail for the Card.
2. Even though users add the images through various mediums like comments, Add attachment button, and, using the + button the latest image attachment in the list (Based on date and time) should be the one shown on the card.