You don’t need to master a photo-editing program—only a few simple tricks.
Crop first. Cut a picture down to just the part you want before retouching, because even a “small spot of deep black or bright color” can “throw a program’s ability to balance an image’s light or color levels.”
See the light. “Reduce the brightness and increase the contrast” when there’s too much flash. When there’s not enough light, do the opposite.
Focus on the eyes. If you don’t have red-eye reduction, “zoom in on the offending eyes until you see the individual pixels.” Then “dab at the red eye” with your program’s airbrush or “desaturation” tool. It drains the color, but “boring gray beats demonic red.”
Fade out. If you can’t get rid of unnatural shades caused by bad lighting, there’s always a “last resort”: Convert the image to black and white.
Source: Popular Mechanics