Make background shown in the preview when moving images

[Yoxem]

Because of the special demand of the video I have to edit, I have to move some images frequently to hide some parts of the video that is not appropriate to be shown in public (i.e. license plate of some cars in the video) accurately.
However, when I adjust the position of the images, the background video of the preview block in "Layout" label of "Clip Property" window (screenshot of this kind of the label: http://www.openshotusers.com/help/figures/clip_properties_tab4.png) is not shown--- the preview block is black. So I have to take many "trial and errors" to move the image clip properly. I hope the developers of the software can make the lower clips able to display in the preview block.

Other imfomation:
OS: Ubuntu 10.10,
openshot ver 1.3
Install Method: PPA