I have the same issue starting at iOS 6, and found the craziest workaround:
Create one more UIImageView of the same dimension. Then add the navigationImage as a subview to that. In my case this will prevent the auto resizing.
UIImageView *workaroundImageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 98, 34)];
[workaroundImageView addSubview:navigationImage];
self.navigationItem.titleView=workaroundImageView;
If anyone finds a proper solution or an explanation please post it.