Open your main project in GIMP
Once it's open, open the directory/folder where is the picture you want to add to your project and click 'n drag that picture you want to put in, inside the canvas of the main project and drop it (or you can drop it directly in the layer dock dialogue),
Done you got your picture in your main project as a layer
Once it's open, open the directory/folder where is the picture you want to add to your project and click 'n drag that picture you want to put in, inside the canvas of the main project and drop it (or you can drop it directly in the layer dock dialogue),
Done you got your picture in your main project as a layer