The cover art shows Harry and Dumbledore on the front, wands out, surrounded by lots of swirly fire. The back shows a birdbath or pillar with a green glow up top. Decoratively, around, is the image of a younger hand linked in arm-wrestling position with an older hand, with fire loops surrounding the place where the hands are joined.
Az's speculation included the idea that Dumbledore was the Half-Blood Prince, and furthermore, that the birdbath-thing on the back was the Pillar of Storge, because that was a title that this book might have been called. She thought that the hands were Harry's and Dumbledore's. She mentioned that, linguistically, "storge" was the type of love that a parent had for a child.
I think that Harry and the Dursleys aren't going to be together anymore after this book. Harry is getting too old and too powerful to put up with their abuse safely for anyone concerned, and yet he has to be with a relative, or he's not protected from Voldemort. I think that the Pillar of Storge is a magical device that will allow Dumbledore to formally adopt Harry, making Dumbledore and Harry indistinguishable from blood relatives in all the ways that magically or otherwise count.