The main problem with the suggestion would seem to be the placement: Changing the top of the page is most likely to cause confusion. (If it's also a large portion of the page, it's even worse.)
A similar but better option would be to make sure there is a consistent top-of-page image or text area (even a small one) which a user can see as confirmation that yes, this is the right/same page (don't violate their expectations with the very first thing on which they will fixate), and have the variable illustration area follow that.
A bit of thought into making the top area communicate that the entire page and not just the top area is "the same" would also help.
Of course, if you can put the variable illustration even farther down the page (maybe the bottom of the area visible before any scrolling), that would be better still, as the user would take in more consistency in the more important, upper areas to counteract the oddity lower on the page.