Well, I'm new to PhotoStage myself but I'll give it a shot.
When you look at the HELP file at you see page 69 http://help.nchsoftware.com/help/en/photostage/win/help.pdf:
"Background Image Widescreen Fit Select the aspect ratio mode for the menu’s background image.
● None: The background image will maintain its current aspect ratio regardless of the video output.
● Crop: The background image will be cropped to match the aspect ratio of the video output. Parts of your background image may be cut off if the aspect ratio does not match.
● Letterbox: A black background will display behind the image to create the correct aspect ratio to match the video output.
● Stretch: The background image will be stretched horizontally or vertically to match the aspect ratio of the video output"
What I don't see on mine, is the NONE option. I suspect that it could be because of the resolution of your pictures and/or the setting in File->Options->Media Files->Snapshot Resolution.
What PS likely needs to do is match the resolution of the media to resolution of the EXPORT video. That is why you have the choice.
I just did a test with Export to Resolution "1920 x 1080 - 1080p HD" selecting LETTERBOX and it produced a good result. It does not show the letter box format you see on the Export.
I'd recommend trying your desired Export Resolution with Letterbox and see if that works for you.