Updated May 2012.
The nature of Raster Graphics Editors like GIMP means that inevitably every user will need to have more than layer open at any point. Though this can be confusing at first it soon becomes apparent that the concept is rather simple and yet at the same time it is crucial to getting any sort of positive result from the program. What is discussed here applies for all the software packages for the most part, though there may be some differences in terminology, hotkeys and in Paint Shop Pro's case the way layers are shown in the interface.
The first thing we need to get our heads around is what exactly a layer is. The workspace appears to show you an image, however this is not entirely true; image the workspace as a desk that will have a photograph on it. Creating a new layer is the same as preparing to put another photograph or item over the top of it, and pasting in another picture or segment of an image is like placing another piece of paper over the other image.
To get a rough idea of this open an image with GIMP, then open another in a second window. In one image press Control and ‘A' to select all, then Control and ‘C' to copy it, before moving across to the other image you have open. Press Control and ‘V' to paste a copy of the other image in. If you select the Move tool by pressing the ‘M' key you can slide this new Layer around the desk as you wish, positioning it as you like. You can move any layer about as often as you like, all you have to do is select the layer you want to move in the Layers pane by left-clicking it and then using the Move tool to move it.
A feature of GIMP is that it doesn't automatically paste objects into the active layer, and neither does it automatically create a new layer for them; instead it creates a placeholder for it called ‘Floating Selection' allowing the user to decide what happens. You can choose whether you wish to keep the pasted item as an individual layer, or if you have positioned it and finished playing with it you can merge it into the layer you position directly beneath it. To merge it into an existing layer just right-click it and select ‘Anchor Layer', but here I want to keep it as a separate layer so right-click and select ‘New Layer' instead; the layer will now change its name and icon to a thumbnail of the image and ‘Pasted Layer'.
Now we'll look at the Layers Pane itself; in here you will see a list of all the layers in your image listed vertically. The layer at the top is just like the piece of paper in the top of a pile; it is this one you will see most easily, and it is only where this piece is not covering that other layers will show. You can change the order of the image by dragging the layers within this pane using the left mouse button, so practise this by grabbing the bottom layer and moving it over the layer we just pasted in. Even though both layers are still visible the one that has been moved to the bottom is no longer showing since the Background layer is fully obscuring it. You can also press the up and downward pointing arrow buttons at the bottom of the layer pane to move a selected layer up and down if you like.
Another basic practise with layers is to rename a layer, so to do this we double-click the layer's name so that it is highlighted and type a new name for it. This is important for more elaborate projects where dozens of layers can get involved quite easily.
Duplicate The Background Layer
One of the most important habits to get into is to always duplicate the background layer of the image you plan to modify extensively. This means that if you want to create a before and after comparison of the image or want to revert to the original image due to a mistake you can do this easily by deleting the modified layer; what this does, in terms of our desk analogy, is create a copy of a photograph in case it doesn't look good once we cover it in paint and filters. Then once we have decided it is rubbish, instead of losing the photograph we can just bring out our copy and throw away the version we don't like.
You duplicate a layer by right-clicking it and selecting ‘Duplicate', do this to your background layer now, and once you have done this right-click the ‘Copy of Background' layer and select ‘Delete'. You can also use the Duplicate and Delete buttons in GIMP, though these are not present in all software packages. I have highlighted the duplicate button in green and the delete button in red in this screenshot so you know where they are.
You can also create new channels to add your own items, meaning that the feature can be used for creative tasks such as digital drawing as well as pasting items into images. To do this select the icon that looks like a blank piece of paper in the Layers Pane buttons, highlighted yellow in this screenshot. When you choose to create a new layer you will be presented with a new window asking you for a name for the layer, the size (which is set to the current canvas size by default) and the background colour. By default the background colour is transparent, which means whatever you don't fill will with colour will allow the background to show through, and if you do choose to add a background colour to the layer at this point it can be white or whatever colour you currently have assigned as the background and foreground colours of your palette.
Another important feature is the use of masks, which is covered in more detail in another tutorial written using Paint Shop Pro, though the principle remains the same.
This tool basically allows you to create selective and non-destructive transparencies in layers; for a quick look at them right-click a layer and select ‘Add Layer Mask'. Leave the options in the new window as they are and select ‘Add'. Now use the brush tool to paint a black line across the layer with the mask; this area will become transparent, to reverse it simply paint over this area again with white.
The other method for creating these sorts of transparencies is with the Eraser tool, but this cannot be undone as easily and is a destructive editing technique (which is always a bad thing). If you look at the Layers Pane the thumbnail for the layer mask will have a black and white representation of the mask currently being applied to each layer for you to check easily.
Above the area where the layers are listed there is a slider for controlling the opacity of each layer. This slider shows a numeric value to the side of it which is the current opacity (default 100) of each layer as a percentage; 100 is fully opaque and 0 is transparent. Move the slider right and left to watch the layer you selected become more and less visible accordingly, this can be done to every layer in your image.
At the very top of the layers Pane is a Mode option that is set to ‘Normal' by default. If you click the arrow to the side of this you will see a drop-down menu of all the layer modes available, a technical guide to exactly what each one including the calculations it performs is available on the GIMP website along with the calculations the software performs for each.
However the best way to learn is to follow tutorials and just try each layer mode out and think about what you might use it for; some are for more technical purposes and will rarely (if ever) be used by most people, but most of them are useful for a variety of tasks. Some lighten the image, others darken it and others transfer the colour from one layer and apply it to another.
That should cover the basics of what exactly layers are and how they can be used and modified, but the best way to learn any of these things is through practise and experimentation. Have a mess around with some photos for a couple of hours; the initial results won't be breathtaking but eventually you should be capable of doing all these things as if they were second nature.
Once you have a good understanding of layers have a go at some of the other tutorials in the techniques section and you'll be surprised what you can do with very little prior experience.