GIMP#12 Add-A-Ghost (Gimp 2.8)
Two interesting things to take away from this lesson are the isolation of the image and the use of the Difference Clouds from the Filters - Render - Clouds - Difference Clouds.
Isolating the image. While there are other ways to carve out a figure from a background, when there is a high contrast between the figure and a relatively solid background, using the Fuzzy Select, which selects contiguous areas of the same color, and should not be confused with the Select by Color, which selects all color in the image. The reason this is important is that there are often small patches of the background color in the eyes, for example. The Fuzzy Select is safer.
When you try the selection, it may decide that the color is not all the same and only select part of the background. This is where the Threshold setting matters. The higher the percentage, the more variations in shade and lightness will match. Go too far and parts of your figure will be selected, but if that happens just dial it down and select again.
Difference clouds: While the difference clouds worked well in this image, remember that while you still have the selection, you can tweak the contrast, hues and brightness generated. The goal here is a spectral image. Also, the larger values that you use for Grow and Feather, the more the effect will be gradual.
Stock image of woman and background from http://fav.me/d2eql1k courtesy of Neriah-stock.deviantart.com. Stock image for ghost at http://fav.me/d59inen courtesy of faestock.deviantart.com