Collection items hold 2 types of information, data and style. An item's style information is how the item will be displayed where the data information is what will be displayed.
When editing the template for a collection item like a menu button or gallery image you can set a style component to show the information held in a data field.
For example you can add an image element to the template and then set it to display the icon data field. The image will now display the icon you set for each item.