GIMP is a versatile image manipulation program full of useful tools for touching up photos. Two of these tools are particularly effective for getting rid of shadows in an image. For small shadows, such as under a chin, you can use the Burn/Dodge Tool to burn out the shadow. For larger, darker shadows, you can use the Healing Tool to cover over the shadow with a sample taken from another portion of the shadow. This is especially good for getting rid of shadows behind someone, caused by a bright flash.
Using the Dodge/Burn Tool
Open an image file with Gimp. Select the Dodge/Burn Tool. This is the last icon displayed in the Toolbox.
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Adjust the size of the brush as needed for the photo in the Tool Options Box. Set the Opacity at "100," using a "Circle Fuzzy" brush. Select "Dodge" as the type and "Midtones" as the range. Use a low exposure of "20."
Zoom into the photo so you can get the tool right on the lines of the shadow.
Click on the shadow and drag the tool across the shadow while holding down the mouse button. Adjust the settings for each photo. If you find the tool is ignoring some portions of the shadow, change the Type to "Shadows" for the darkest areas, or "Highlights" for the lightest.
Using the Healing Tool
Open an image file with Gimp. Select the "Healing Tool" from the Toolbox. It is shaped like two crossed bandages.
Set the Mode as "Normal" in the Tool Options Box. Set the Opacity at "100," the Scale at "3.5," and the Length at "100 px." Select "Fade Out" to give the tool a soft edge. Select "None" for the Alignment.
Zoom into the photo and examine the area around the shadow you can use to replace the shadow. Place the tool on this clean area. Hold down the "Ctrl" key and click the mouse to take a sample of that area. You will see a cross-hair icon where you clicked.
Move the tool over the shadow, and then drag it across the shadow while holding down the mouse button. The cross-hairs on the sample area will move parallel to the tool, showing you what is replacing the shadow.
Release the mouse button when the cross-hairs approach an area not suitable for replacing the shadow. The cross-hairs will return to the original sample spot. Click on the shadow and resume healing the shadow.