"Use your imagination!"
The usual perfective/imperfective difference. "użyć" should rather be one-time, it makes sense here as "Use your imagination, now!". "używać" would be in general - "Geez, it's like you don't think at all, use your imagination from time to time!".
I guess there's no reason to reject "używaj" (added now), although it wouldn't be a common thing to say.