It is appropriate that Harry and Dumbledore dominate the cover as they dominate the story. Dumbledore has been a strong presence since the first page of the first book, but in this book he gets more screen time and much more depth.
Reflecting on the changing tone of the books as the series has progressed, it seems to me that they become darker as Harry becomes older, in some way reflecting the passage from child to adolescent and finally to almost adult. I'm not sure that this is a conscious thing from Rowling but the later books are more young adult in keeping with the age of the protagonists. As anyone who has read young adult fiction it isn't generally a pleasant place to dwell for too long!
The second last book in the series is an extended treasure hunt and has an ending that I didn't see coming and that I didn't accept until I had read the last book. As one has passages in a book that one doesn't like but has to pass through to get to the good parts, so this book is a passage to more cheerful things!