IMO - journaling helps you with your ability to express yourself. It doesn't, IMO, help with imagination in all cases. Imagination means that you are able to picture something that isn't present. If you're writing about things from your past, say an incident you were part of, then no you are not using your imagination.
However, if you are writing about something that you want to create, then you are using your imagination.
I think the key here is that you are writing about something that doesn't exist. Something that you either wish did exist, and are describing it in a fairly detailed manner (how it looks, feels, functions, etc.). That is where imagination comes into play.
But, even if you aren't writing about something that doesn't exist, you can still be accomplishing a lot with your journaling. For example, I've been going through a lot of memories from my past. As I've gone through those memories I've had some interesting thoughts about how I perceived things at the time, versus how I perceive them now.
You also can find new ways of expressing yourself. There are things that as I have written about them in my journal, I've writing about them in ways that I wouldn't have written about them in a more public setting. However, now I find that some of those expressions are starting to come through in my public writing — and it's for the better.