Note that "icon" is sometimes misleading, and should be interpreted in the context of the entity. For instance, in Destiny 1 the DestinyRecordBookDefinition's icon was a big picture of a book.
But usually, it will be a small square image that you can use as... well, an icon.
They are currently represented as 96px x 96px images.
Many Destiny*Definition contracts - the "first order" entities of Destiny that have their own tables in the Manifest Database - also have displayable information. This is the base class for that display information.