Example of seamless object incrustation on a background image, with the G'MIC plug-in for GIMP.
Here I show how you can merge an object onto a background image without visible seams, and without having to do a very precise selection. This is done in GIMP with the help of the G'MIC plug-in (http://gmic.sourceforge.net/gimp.shtml).
More particularly, I use the G'MIC filter 'Layers / Blend [seamless]', that implements the Poisson Editing technique by Perez-etal to merge a top and a background layer together.
This video is in real-time, so you can see that there are no tricks and that the edit is really simple and quick to do !