With any serious image work, you will find layers essential.

The following diagram  from "Grokking with The Gimp" shows this concept very well:

This image is in fact made of 3 layers, as below:

As you can see, each layer is a separate entity but these layers combine to form a single picture.

Layer information can be found by choosing Layers > Layers, Channels and Paths. The information looks like this:

The Layers Palette also tells you what's visible in your image: if the Eye Icon is visible, the corresponding layer is visible in the image window. The Eye icon can be toggled off by clicking on it. This makes the corresponding layer invisible in the image window. Toggling it again brings the layer back.

To the right of the Eye icon you may see a Layer Link icon. Clicking in this area makes a four-way arrow appear, as shown in two of Prairie Pranks'  layer strips. When this icon is activated for several layers at once, they are linked together as far as the Move tool is concerned. These layers will now move together as a single unit.

To the right of the thumbnail is the Layer Title Area, which, for a new image, is named Background by default. The title can be changed by double-clicking in the Layer Title Area. This brings up a dialog box where the new title can be entered.

The middle layer strip in is highlighted in blue, indicating that it is the active layer.  Things you do in the image window are applied to this layer.  Repeat after me: The GIMP tools and filters are applied to the active layer!!  Only one layer can be active at a time.

The Opacity slider is useful to control how lower layers can be seen through higher ones. Here the middle layer strip is highlighted, indicating that this layer, containing the small white rectangle, is active. The Opacity slider is set to 60.0% for this layer. This means that the layer below can be seen through the white rectangle because the rectangle's layer is only 60% opaque (partially solid). 

We are now going to re-open our Gardening image, and add a new layer, called Logo. In this layer, we will place a school logo so that our gardening image appears to be semi-transparent in front of this logo.

Firstly we will create a new layer called Logo. Use www.education.tas.gov.au/ and find a school with a logo, and save that logo somewhere convenient. Open the logo image in The Gimp, and copy it to the existing layer of your gardening image. With the Layer Palette visible, you should see that a new layer appears called Floating Selection. Make this into a new layer, call it Logo (right-click, then edit Layer Attributes) and all should be well  

Using the Layer Palette, adjust opacity levels, and if necessary relocate or adjust the logo so it does not obscure major parts of the image. To set the garden in front of the logo, the garden layer will need to be promoted in front of the logo, which in turn means it must have an alpha channel to control transparency. 

Normally, the Background Layer of an image has no transparency properties, because it starts life as a single-layer image where transparency makes little sense. You can add the ability to have transparency by right-clicking on that layer in the layer palette, and selecting Add Alpha Channel.  Once the layer is capable of moving in front of another, you can move it using the Layer Up/ Layer Down arrows in the Layer Palette. Remember: you will only see the layers marked with an Eye, but the actions you perform will be on the Active layer.  This is almost certain to prove confusing so keep your Layers Palette visible.

Finally, you can merge your layers into one (if you are sure you no longer want to keep layer information) by using Layers > Merge Visible Layers.

Copying and Pasting: The Gimp vs Microsoft Windows.

As The Gimp was developed as a multi-platform product, it has its own internal clipboard where things are copied during a copy/paste operation. This is not the same as the clipboard used by Microsoft Windows, even though they have the same name and do the same sort of job. The Gimp does however have a Copy to Clipboard/ Paste from Clipboard menu item that talks to the Windows clipboard.

Important copyright information

This page is based on the material from section 2.1 of Grocking the Gimp by Carey Bunks.
Copyright (c) 2000 New Riders Publishing. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/).  Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder. Distribution of the work or derivative of the work in any standard (paper) book form is prohibited unless prior permission is obtained from the copyright holder. Other forms of distribution including CD-ROM, electronic, and magnetic media are permitted.