Hello,
I am new to GIMP-scripting, but advanced in multiple programming languages. I managed to create a V3 script-fu function creating an image with several text layers, a few layers to draw on and a layer containing the picture. All layers can be scaled an placed individually as needed, but:
the picture in my layer does not scale/resize. The layer contour is resized however.
My relevant code lines:
(set! theFoto (gimp-file-load-layer 0 theImage theFile))
(gimp-image-insert-layer theImage theFoto 0 0)
(gimp-layer-resize theFoto 3000 1600 0 0)

What is it that I am missing here?

Perhaps try,

Resize and scale are two different things.
Resize changes the layer's size (with, height, offsets) as you have already seen, it doesn't scale the layer's contents.
If you want to scale use for example gimp-layer-scale.

@teapot: Thnkx ! Thatś what I needed.

@rod: I only needed 1 layer to scale, not the whole image.