Welcome to the help page for Infobox.
An infobox is used in certain articles of certain categories to list all important information at the beginning. You can view such an infobox in use in the articles Caves (of the type "level") and Bats (of the type "enemy") as examples.
In such a case no main image for the article is needed, as it is already integrated into the infobox.
Images used in infoboxes should always have a 4:3 (width:height) ratio, to guarantee it is displayed OK.