Blending in Images into Mockups automatically

[kaiserart]

Hello Guys,

I am currently setting up an online shop where i have mockups of digital art.
I am relatively new to gimp but i am fairly trusted with scripting/coding.
I already managed to write a plugin which rescales a file to several formats.

Now i want to fit an image into the empty mockups. Since i dont want to waste hours doing that manually, i want to automate this.

Example:
1. I have my blank Mockup Image
2. I have my image which does not yet fit into the mockup space
3. Now i have to insert the image into the mockup image
4. Now i have to rescale and move the image to the mockup space and make it blend in perfectly
5. Save the mockup with the inserted image

I want to automate this procedure so i can chose a set of images and then they get inserted into different mockups automatically.
Is there any function/plugin like this?
If no is it possible to create a fu-python plugin myself to achieve this?
Basically the mockup does not change. The empty space will always have the same position and size. I just have to fit the image to it.

Thanks in advance if anyone can help me!

Best Regards
Daniel