User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7742465-20130522145545/@comment-3146930-20130526005556

If I had to guess, I think it is because Polyphemus is watching them (as displayed by the large eye in the background) and the cover makes it seem like he has been watching and waiting for them this entire time. However, in reality he was blind and didn't even care about them until they tried to take the Golden Fleece.