No matter how careful you are in composing your image, there are bound to be flaws - be they acne, lens damage, or even a careless smudge. This article provides an overview of some of the tools that GIMP provides in fixing these flaws, and how to use them.
GIMP, if you aren't already familiar with it, is a completely free and absolutely fantastic open source photo editing software, one that rivals even Photoshop. GIMP provides you with many easy to use tools to let you make your picture just that much more perfect. Here's an overview:
This little Band-Aid icon is probably your first place to go for editing out blemishes. The best thing about this tool is that it will replicate the texture over where the blemish is, making it almost impossible to tell that there was any editing done at all. To use, activate this icon and, holding down CTRL (though this is dependent on your operating system settings), select a place near the blemish that has approximately the same texture that you want to replace the blemish. Now just brush away at the blemish like you would with any other brush, and watch the blemish disappear! Make sure you have a rounded, fuzzy-edged brush selected to minimize any obviously artificial sharp edges.
If the heal tool doesn't look like it will do the trick, or if there are some residual editing marks left over from the process, then using the airbrush is probably your next step. Using the eyedropper tool, select the color that you want to replace the blemish – don't worry, the area won't be completely this color. Make sure you have a rounded, fuzzy edged brush selected to prevent those same sharp edges.
If there's a subtle color gradient, then you can repeat this process to create as exact a gradient effect as you like—but you don't have to make it precise, because there are other tools yet that can be used.
The smudge tool is the next step in blemish reduction, if the previous steps haven't already taken care of it satisfactorily by now. The smudge tool can be used to, as the name suggests, smudge boundaries between colors so that it appears less contrived and smoother. This is a great way to clean up after an airbrush or heal job.
Similar to the smudge tool, the blur tool can be used to clean up after the airbrush. It also works very effectively to make skin look smoother and produce a subtle, healthy glow when used with care. As with the smudge tools, make sure that you follow the lines of the object that you're editing to make the editing less conspicuous and the image more natural.
This applies to all tools: apply with caution! There is a tendency to over edit, especially with the blur tool. While blurring is a wonderful way of disguising flaws in an image, too much blur can result in an undesirable loss of image sharpness, and even stick out all the more in an image than the original flaw. There's a balance to be found, one that requires practice and a developing eye for photo editing.
If the blemish is just a matter of light or darkness, then the dodge/burn tool might be what you need. As with the others, make sure you have a fuzzy-edged, rounded brush. Don't go overboard with it either: keep the percentage to the teens and go over the flaw with care.
The more attention you pay to detail, the better your image—and its new lack of blemishes—will turn out. You're limited only by the amount of time you wish to spend working with these tools.