It is important for covers to convey a feeling. It would make no sense to have a book about an ice planet and see a sunny beach on the cover. This should hold true for any book.
Ice Planet is a composite that uses a group of frost covered trees, a frozen lake, some hills for a backdrop, and a sky at sunset with a ringed planet. The two girls with their dog are seemingly in awe, which is a good way to put the potential buyer of such a book in awe, and curious of the unusual setting. Book covers should convey something about the content.