"The GIMP is the GNU Image Manipulation Program. It is a freely distributed
piece of software for such tasks as photo retouching, image composition and
image authoring. It works on many operating systems, in many languages."
For windows, see http://gimp-win.sourceforge.net/

First Image (Background)	Second Image (Foreground)	Third Image (Intelligent Scissor Result -- with Contour)	Fourth Image (Intelligent Scissor Result :--Mask)	Fifth Image (Artifact)

The basic composition process using GIMP is listed below and the GIMP command is shown in bold.

Comments about each step are added in the end of each step.

Open the 1st .image //Open the 1st .image as Background

Open as Layer the 2nd image. //Open the 2nd image as Layer 1

Open as Layer the 4th image //Open the 4th image as Layer 2

Layer->Mask->Add Layer Mask ->Grayscale Copy of Layer ->OK //Add a Mask which is a grayscale copy of the 4th image for Layer 2

Layer->Mask->Mask To Selection //Copy the Mask to the Selection buffer

Layer->Delete Layer //Delete Layer 2

Layer->Mask->Add Layer Mask ->Selection ->OK //Add a Mask which is a copy of the Selection buffer for Layer 1. In other words, add a Mask which is exactly the 4th image for Layer 1

Image-> Flatten Image //Combine Background and Layer 1 together to form a new image in the same layer

File->Save as the 5th image //Save the new image as the 5th image

The above composition process is just a very simple example of compositing, and
doesn't explore the more advanced features in GIMP. 

You are welcome to experiment them in your artifact.

See GIMP Documentation and Tutorial for more details.