If you mean you want the image to be the whole screen but still have the turn off computer you need to crop image.
For ease of explaination will use 1024x768 image size. Open image in graphics software, rect. select 80x1024 from bottom of image, crop image to this, file save as Bitmap_201.bmp in logon folder, after saved undo changes in graphics sofware now rect select 1024x688 from top of image, crop to this, save back as Bitmap_100. In logonstudio editor set bottom panel height to 80, don't backspace out all numbers or editor will crash, set style to stretched.
There is no transparency setting as it would not do anything, image is applied between top and bottom panels they do not cover it.