I need help with creating a map in Gimp

[theignis23]

I am very new to Gimp and these forums. I recently got a homework. I have to create a map made out of images. I attached an example image below. I have made few images before, so I thought, that this won't be a problem. I tried cropping a state out of the original map and adding a layer below. Then I coppied an image in place, but it always ended up on top layer.
I didn't know a better community to ask, so here I am. Is there a way to create a map like this in Gimp? Please accept my begginerness.