I was surprised by my image--a pair of tail light disappearing around a bend. Seemed very mundane and a little cliched. But then I had them all write scenes that lead up to their image and the scene I wrote pleased me and surprised me and felt right. And it led to the tail lights disappearing.
Also, I think it's exciting to have the ending to head toward. Even if it doesn't stay the ending. It has a kind of weight that can pull you along when you're struggling with the middle, etc.
So try it. Write an image that a novel can head toward and then aim at it. Might even be the first thing you write in a novel.