YMMV, but we did plenty of testing for one site I currently work on with a heavy mobile access focus, and it wasn't even close. We could have added hundreds of extra KB in a CSS file and still beaten even a single additional image file that needed a whole extra HTTP round trip for a visitor with a typical intermittent mobile connection (poor reception, on the move, etc.). The multiple levels of set-up required in that context can easily dominate the overall time to seeing a useful page.

Of course it's less of a dramatic comparison if you don't have all that set-up overhead to worry about, but then the time to download an extra 200K of CSS is negligible on basically any broadband or stable 3G or better connection today, so we don't tend to worry much about that sort of situation. In practice, our decision on how to transmit medium-sized images is often based on cache-related factors.