Ah. I see (I think...)
When creating a new image comparison mask, the reference image is provided as a guide. (Otherwise, how would you know what to include or exclude from your mask if you can't see what in the original image to leave in or cut out?) What we are doing is creating the mask. We are not saving an image to the store, we are saving the mask and we want to see a preview of the mask before saving.
However, I think you're expecting to see the masked image as well as an example of what would happen when the mask is applied to the image. If so, I think that is a great idea! It would certainly make it obvious what would happen during the comparison.
Please make the suggestion to our Feature Request forum:
http://community.smartbear.com/t5/TestComplete-Feature-Requests/idb-p/TestXCompleteFeatureRequests