A thumbnail is a preview of a game. In total, ten thumbnails can be used in just one game. The three types of thumbnails a player can select are an image uploaded from a device (FREE), a YouTube video (500 Robux), and an auto-generated thumbnail, which is provided to a user for free.
The thumbnail was initially added to Roblox during its launch in September 2006. Since then, the default thumbnail has gone through a number of changes, and price was changed from 10 Robux to free on September 7, 2018.