Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 216

3.6.5 Step 4: Adding a New Object or
Layer (Sky) to an Image
GIMP offers several alternatives for importing images as layers and combining
several different image files into a single file. Importing images is pretty
easy: Simply drag and drop a layer from the Layers dialog on another image
window. However, it is a good idea to adjust the size and resolution of the
image layers you intend to import so they correspond to the target image's
size and resolution. When importing images, only the layers are copied. The
original images remain unchanged.
Figure 3.41
Dragging a layer