Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 298

In that case, the image layers would have to be aligned above each other as
in the method described in section 3.13.2. The overlying layers would have
to be made invisible, and in the other layer, the opacity would have to be
reduced to 40% in order to align the undermost layer. After the first two layers
are aligned, the opacity of those layers would be raised to 100% again and the
procedure should be repeated with the next layer.
Label your layers in the image as details-highlights , median , and details-
shadows . All three layers are missing an alpha channel, so add one for each
using the context menu (right-click on the layer and choose Add Alpha
Channel ). Then save your image as ldrcollage.xcf in the XCF file format.
Figure 3.112
The image in XCF file format and the corresponding layers in the dock.