convert -size 100x100 plasma:fractal plasma:fractal -append out.psd

> (gimp-image-new 200 200 0)
(1)
> (gimp-layer-new 1 200 200 0 "layer-1" 100 0)
(2)
> (gimp-layer-new 1 200 200 0 "layer-2" 100 0)
(3)
> (file-psd-save 0 1 0 "test.psd" "test.psd" 0 0)
> (gimp-image-add-layer 1 2 -1)
> (gimp-image-add-layer 1 3 -1)
> (file-psd-save 0 1 1 "test.psd" "test.psd" 0 0)

gimp -i -b '(your-script-name "test.psd" 200 200)' -b '(gimp-quit 0)'

convert -size 100x100 -alpha set plasma:fractal -alpha set plasma:fractal -adjoin out.psd