Complex assets created using the Builders within BAM! are given a default thumbnail preview image based on the file type and other factors, but the user can change those preview thumbnail images by hovering over the upper right corner of an asset card and interacting with the Thumbnail Manager:
Changing an Asset's Thumbnail to a Custom Image
Selecting Custom from Thumbnail Manager will open the Upload Custom Thumbnail box and allow you to select a new image for that particular asset's thumbnail preview:
Changing an Asset's Thumbnail to an Icon
Selecting Icon from the Thumbnail Manager will replace that asset's thumbnail preview image with an icon representing the asset's file type:
Changing an Asset's Thumbnail to its Original Image
Selecting Original from the Thumbnail Manager will set that asset's thumbnail preview back to the original image.