"Image song", if you are not familiar, is a Japanese transmedia concept for music that conveys the idea of an anime character, etc.; unlike character themes, they are typically not featured in the actual show. So that just means the disc will have new songs written for the manga, same as all previous CDs bundled with Touhou books.
I want to say there's still a chance the normal edition will also contain the CD, and it's only the artbook that's special edition-exclusive, but it's more likely we really need to pay an extra 1000 yen for the CD and an artbook not even big enough for Moe Harukawa's larger artworks. The reign of Kadokawa has begun.
Also from the leaker are some spoilers of Vol. 5's character design drafts:
http://i.imgur.com/zI7S1aU.jpg
http://i.imgur.com/u21KLx9.jpg