Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 183

So far we have been editing images without bothering with the features of
the layer. Every image that you open with GIMP has a background layer. The
background has certain features:
• First of all, background layers are always called background. The name
appears in bold in the Layers dialog.
• Background layers are not movable within the layer stack and always lie at
the bottom of the stack.
• If you erase or cut something out of a selection of your background layer,
it will appear covered in the chosen background color in the program. The
reason is that the background layer does not have an alpha channel to enable
a transparency. Actually, when you are erasing, cutting, or deleting something
on a background layer, the color of the paper (background color) lights up.
If you want to have transparent surfaces in a background layer, you have to
allocate an alpha channel over the context menu of your Layers dialog. Right-
click on the layer in the dock, and then simply select Add Alpha Channel from the
context menu. Then you can move the layer around in the Layers dialog or place
other layers under this layer in the stack. In the following exercises, you will be
working with these options.
3.3.4 Working with Several Images—
Inserting Layers from Another Image
You can view the layers of an active image in the Layers dialog. If you have
several images open at the same time, the active image will be the one in the
foreground; the active image will also have a highlighted blue title bar.
When working with several images, you can easily drag and drop layers
from the Layers dialog of the active image to the image window of the other
picture. (In the Layers dialog, click the desired layer of the first image, and while
holding the left mouse button, drag it onto the image window of the second
image and release the mouse button.)
However, you can also insert a new picture as a new layer in an already
opened image. Therefore, you must select your image from File > Open as Layers .
Your picture will be opened as a new layer in the already opened image.
In both cases it is essential that the images have approximately the same
size and resolution. It's not hard to scale down the inserted layer. However, if
the inserted layer is considerably smaller than the original image, you must
enlarge it. This will have a significant effect on the quality as the inserted image
is recomputed and may appear blurred.
So far in the editing you have done, you didn't need to have any prior
knowledge about layers. In following examples, you will learn how to work with
layers for the first time.