GIMP: Add padding to multiple images

I scanned a couple pages and edited them a little so the look neat.
But as I wanted to reprint them I noticed that I cropped the border from these images so that the printer won't print the whole content.

I have a Brother printer which has Linux drivers but somehow the scaling option does not work.
So I thought to scale the PDF (in which I have converted these scans) but the printer driver scales them back to fit the page. (If I disable the scale-to-fit option it becomes garbage)

So I thought that I could do some script-fu to resize these images with a couple of lines and add some padding for the printer. But I have no clue how to do this.

Here's my first attempt:

(define (resize-image filename-in filename-out )
  (let* ((image    (car (gimp-file-load RUN-NONINTERACTIVE filename-in "")))
         (drawable (car (gimp-image-active-drawable image)))
        )
     (gimp-image-resize image 2728 3819 124 173) 
     (gimp-file-save   RUN-NONINTERACTIVE image drawable filename-out "")
  )
)

This does not work. The image simply remains unchanged.

My page is A4 with 2480x3472, so I thought to add 10% to the width so it becomes 2728x3819 and set the offset to 5% so the content is centered (with offset values 124 and 173).

Solution

Wnen you resize the image you are just extending the canvas but not the layer. And gimp-file-save only saves the active "drawable" (layer in your case), so you just save the same image. What you have to do is either:

add a white layer at the bottom of the layer stack
flatten the image
save the result

remove the layer's alpha channel (assuming the background paint color is white)
extend the layer to cover the canvas (the extension will be filled with white): gimp-layer-resize-to-image-size
save the result