Hi cafink,
I guess it's a bug. I had the same problem.
To display the slider properly, you can modify the file simplest.wps in the theme.
Replace:
%pb(0,0,-,-,-, slider, slider_image)
With:
%pb(0,0,-,8,-, slider, slider_image)
(8 is the height of the slider in pixels).
You should see it entirely now! Hope it helps.