In this tutorial, we will teach you how to do a face swap in GIMP. For this tutorial, we will be working on two free stock images. Both of them can be downloaded from the following links:
Male Models:
http://www.freeimages.com/photo/1024098
Black & White:
http://www.freeimages.com/photo/1149181
We will use the male models image as the base and bring the black and white image on top of it to do a face swap. So load both the images in Gimp and let's get started.

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Step 1 -- Resizing the image on top
First of all, resize the top image according to the first image. For that, move to the Toolbox on the left side and select the resizing tool from there. After that, move to the canvas and click on the image.
Once you do that, a new panel will appear on the screen. Over here, reduce the dimensions of the image so that it matches with the dimensions of the original image. Once done, click on the scale button to apply the changes.

Step 2 -- Changing the opacity of the layer
Next, reduce the opacity of the layer by moving to the layers panel and reducing the opacity until the previous layer is visible. With that done, adjust the top image in such a way that the facial features match with the image below. To align the images better, you can pick up the resizing tool from the toolbox and further reduce the size of the image on top and position it accordingly.

Step 3 -- Adding a mask to the layer
Now what we'll be doing is removing the face structure of this image, preserving only the features like the eyes, nose and smile. So first, create a layer mask by right clicking on the layer and selecting the "Add a mask" option. With that done, a new window will open up. Make sure that you select the White, full opacity option there.

Step 4 -- Removing everything except the facial features
Next, pick up the paint brush tool from the toolbox. With that done, do remember to select black for the foreground and white for the background color. After that, head back to the layer menu, click on the top layer and bring its opacity back to 100%.
Now make sure that the layer is selected, and start applying the brush over the image. Just keep applying the brush and remove everything from the hair, the background, the neck etc. Only the face should remain. Remember, we only have to keep the features and swap it with the image behind.
You can use different levels of opacity and further mask off the sharp edges so that it merges with the image behind. This would include the chin, the forehead and everything else except the facial features. Once done, you would have a rough idea of how the image would look like after the face swap.

Step 5 -- Adjusting the color of the top layer
After that, you would need to tweak the colors a bit so that the image blends in well. For that, make sure that the top layer is selected and then open up the color menu and select the curve option.
Over here, you will have to change the straight line to form an upwards curve in order to increase the white balance in the image. Keep adjusting the graph to match the skin tone of the image. With that done, click on the ok and our image is ready.
And that's how you can do a face swap in Gimp.