User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-24339098-20130524013304/@comment-5929284-20130524130121

Although it kind of exploded on tumblr, Rick did actually say in an interview that the cover will give us a hint as to Percabeth's situation in Tartarus. I Here's the link

--> www.usatoday.com/story/life/books/2013/05/07/son-of-sobek-rick-riordan/2139069/

It's the last question :)