In order to edit your photos, you'll need editing software. The most famous, and arguably the best program, is Adobe Photoshop (http://www.photoshop.com/). But Photoshop can cost hundreds of dollars, and many free alternatives exist. For ease of use, try Google's Picasa (http://picasa.google.com/), and for more powerful but complicated software, use GIMP (http://www.gimp.org/). Each program is organized differently, so for simplicity, we'll provide an overview of a few methods that will greatly improve your photos.
An easy first step in the photo editing process is cropping, or cutting out, unwanted parts of the image. Effective cropping will improve the framing of the image, and will focus attention on the subject of the photo. An easy method that will improve the composition of the image is the rule of thirds. The rule of thirds is based on dividing the image into nine equal squares. Positioning the subject on one of the corners of the central square will result in a more dynamic image that is naturally pleasing to the eyes. Most importantly, it takes the main subject of the image away from the center of the frame. In the case of a headshot, in which the subject's face takes up most of the frame, position, for example, the eyes at one of the four corners.
Increasing the contrast of the image can also be a great way of livening up the headshot by minimizing any dull grays in the image, and accentuate the light and dark components. Saturation boosts the colors of the image, making them look more vibrant. However, increasing the contrast and saturation should be done subtly. Add too much contrast and saturation, and the photo will look amateurish.
The clone tool is one of the most powerful tools for editing photos. With cloning, you can remove scars, stray hairs, or other unwanted objects from your photos. The software accomplishes this by copying a similar section near the object you'd like to remove and replaces the object it with the copied sample. Using cloning, you can cover an unwanted facial marks with a copy of the similarly colored, clear skin next to the blemish. Of course, like adding contrast and saturation, cloning, if overdone, can look unprofessional.
The most important things to remember when editing photos is to work slowly, and always save your work. Create a new copy of the original image and work off of that copy, retaining the original in case you lose your work.