That's my bad for not being clear.
The image executable works on image sets that have already been extracted from the resource archives. The briefing.vga file is a resource archive, not an image set. Before you can extract any images, you'll need to extract the resources from the archive. That's where the resource executable comes in.
Do this:
resource -extract-all briefing.vga -o briefing
This will create a new directory called briefing; inside, you'll see a bunch of numbered files without any extensions. Those are the individual resources from briefing.vga. It happens that all of those are image sets. For each one, you can do this:
image -extract-all 0 -prefix 0_
That will pull all of the images out of the file named 0; each image will be named 0_0.png, 0_1.png, etc.