Hi,

I'm sure you've solved the problem, came across this thread on a general search.

The two images shown by Miss Nancy are both needed. The one on the left is the actual lightprobe, small and blurred which produces a smoother render and take less time to analyze, and the one on the right is the background for clarity reason, I don't use Carrara but it should have a slot for both.
 
Hope this help.

Chris.