When setting up an event, you will come across the option to upload a Main Image and a Featured Image.
A Main Image is an element on your event page. It the image in the header.
A Featured Image on the other hand shows on the ticketing home page. It is controlled only by Portal Level users.
The biggest distinguishing note is just the size of image that should be used for either. A main image should be a square and ideally 500 x 500 pixels or larger. A featured image on the other hand, needs to be at least 400 x 1000 pixels.