One of the most common uses for raster graphics applications such as Gimp is the doctoring of portraits to remove imperfections and improve the image. While professionals make their subjects thinner, more tanned, wrinkle-free and sometimes even enlarge certain body parts we aren't going to go as far as that. What we are doing here is touching up a portrait, removing blemishes and just generally cleaning up the image; this will give you valuable experience with a variety of Gimp's tools.
Step 1. Well, firstly we shall address one of the more obvious issues; our model has a few blemishes oh her forehead and the nearside of her face. To combat this, we need to create a new layer (we're editing this using non-destructive techniques as always), so press the New Layer button in the Layers Pane and then rename it to ‘Blemishes'. We also want to create a duplicate of the original Background layer (why will become apparent later); right-click the Background layer and select ‘Duplicate', then rename it as usual.
Step 2. To cover these blemishes we want to use the Clone Stamp tool to cover them with a neutral skin tone. Select the Clone Stamp tool (‘C') and then look at the options in the Settings Pane. Ensure that the Source is set to ‘Image' and then tick the checkbox beneath it ‘Sample Merged'. This means that whatever is visible from all the layers is sampled rather than just the content of the layer you are working in. Set the Brush to a soft-edged one so that the result isn't too clearly defined so as to betray your edits.
Step 3. Change the tool's size using the square bracket keys to increase (‘]') and decrease (‘[‘) it so that it is roughly the size of the blemish you are removing. Hold Control and left click an area nearby and that has as close a tone as possible to the area surrounding the blemish. Once you have the source prepared, left click on the blemish to remove it.
Step 4. Now we want to shape her eyebrows since they are a bit untidy. This is done in pretty much the same manner as removing blemishes; so create a new layer for eyebrow edits and rename it. Then select an area of skin just next to the area you are going to start and begin painting over any hairs that lie outside of the shape that the model has tried to achieve. Unlike with the blemishes, we are mainly going to be holding the left mouse button and drawing a line following the shape that we wish to achieve along the eyebrow instead of merely pressing a button. Every time you release the left mouse button re-set your source area so that it is nearby once more so as to achieve the greatest match between your edit and the actual skin tone around the eyebrow. In order to successfully integrate this edit into your image you may have to reduce the opacity of the layer to soften the effect it has. I reduced mine to around 70% to make the change more subtle. You can use the same technique to clean up hairlines and beards too where necessary.
Step 5. Now we have the basic skin alterations we can now look more closely at the photo as a whole for a minute. Right-click the layers you created for the eyebrow and blemish corrections and select ‘Merge Down' to put them both into the duplicated Background layer. Now correct the exposure of your image and colour of your image if required. All I did to mine was use the Levels dialogue to brighten the image; the model was looking dull as a result of the bright background which contained a lot of white, so at the cost of allowing the white to bloom a little bit more into the model's face I've allowed her to compete with this white for attention. The reason we didn't do this straight away is due to the risk of distortions and loss of image information that can result from this sort of change; this way we have allowed our corrections to contain as much detail as possible so they look more realistic.
Step 6. My model doesn't really need this step since the image is so bright, but another tactic that is often required is whitening eyes. My image will only really benefit from this by it reducing the apparentness of the slight-bloodshot area in her near eye, but it also lightens the eye too so that in darker images the eyes can still stand out, and this is very important given how expressive eyes are in portraiture. Create a new layer and set the Layer Mode to ‘Dodge'. This means that any colour we add acts as a certain strength of the Dodge tool, with pure white making anything brighten to white, and black making no difference. Set a soft-edged paintbrush to a soft-edged tip and select white as your foreground colour before painting the whites of your model's eyes. Of course this result is horrendously unnatural, so you need to reduce the layer's opacity, its better here to not change your image too much than to risk it looking unnatural (unless this is the effect you are after of course). The screenshot here is of a before-and-after comparison; as you can tell the effect is only subtle; I set the transparency to around 15% to achieve this, which should give you an idea of how low a value may be required to get this right.
Step 7. My model isn't really smiling unfortunately, and she seems to have lovely white teeth already, but if you repeat the process you used for the eyes you can whiten your model's teeth if necessary. One thing you need to avoid here, however, is brightening any gaps and dark areas in the mouth as this will make the image look artificial. You also need to be careful when lightening any teeth that are not at the forefront; these should not be whitened as much, if at all, so use a grey shade rather than white to edit these.
Step 8. This is optional, as is most of this tutorial really; it all depends on your image and the effect you are going for. If you want to enhance or alter the colour of your model's eyes draw a selection around one of their Irises using the Freehand Select Tool, then once you have done this, hold shift and start drawing around the next to add to your existing selection. Check the box in the Settings Pane to Feather the edges of you selection. Create a new layer for this change (renaming it as well as usual), and if you want to completely alter the colour of the eyes keep your selection active before entering the duplicate of your Background layer we made earlier and selecting Colour>Desaturate
Step 9. Press Shift and ‘B' to activate the Bucket Fill tool, then select the colour you either want to make her eyes or enhance them. My model has blue-green eyes so I've decided I want to add a bit more of the green to make them that little more turquoise (I'm not completely changing the colour of her eyes, though you might want to). Enter your newest layer whilst you selection is still active and click inside one of your selections, which will fill them with a solid block of your colour. Change the layer mode to Colour and reduce the opacity until the change is believable.
Step 10. Finally, if your model has eyebrows that aren't very dense or appear weak, Press Shift and D to activate the Dodge/Burn tool. Create a Duplicate of the layer you already collapsed all your skin alterations into and rename this one to make sure you can tell the difference; Burning the image as we are about to can easily go wrong, so it's best to have the original image we are burning available should we make a mistake. Hold control to put the tool into Burn mode, then draw over the eyebrows, whilst painting do it in as few individual button presses as possible; if you draw over an area twice using the Burn tool without releasing the left mouse button nothing will change, but if you have released it since you painted that area it will double the strength of the effect you applied. So try not to draw over an area twice unless it is looking particularly pale.
There you have it, a picture with no imperfections.