Group images can be uploaded and cropped on edit group information page. Image cropping allow a group image to look nice for different types of usage:
Large. Crop group image for the top area of a group.
Small hero
Site managers can choose to set all the group top image into a smaller version so all the group content will become more prominent. If this is the case, group managers are provided with another cropping type called 'small hero' for this situation.
Small. Crop group image for the group teaser - an overview block of the group by clicking on which you can reach the group page.