Glad you figured it out. And thanks for pointing out that there's no support for posting photos to a friend's wall. I'll definitely have to look into adding that.
Could you share the code you used with GraphApi.publish() for the rest of the community to learn from until I get it added to the API binding?
I actually already have some work in progress to support a cleaner way of posting to a wall (no matter who owns it, as long as you have permission to do so) and this work should make it easier to add a photo as well. It's just rough work right now (in a local branch), though.
Spring Social Project Lead