You can put images at 3 levels:
- An image for the event
- One image per category
- One image per ticket
The image that appears on the ticket is the one you put in its settings. If you haven't put one, it will be the one of the category to which it belongs. Finally, if there is no image on the category (or it does not belong to a category), this will be the event image.
That being said, you can create a ticket office without using an image. In this case, the ticket image will be a white rectangle.
This is what a ticket looks like with the image: