A headshot is an 8" X 10" glossy photograph that is two-sided. One side has a full facial shot, the other side should have a composite of three or more photos that represent the kind of characters one might be able to portray. Your name and contact info, including any agent info, must be on the composite side, as well as a list of acting credits, if you have any. Professional photos should be taken.
The Full Face Shot
This is technically the headshot photo. This shot will be the dominant representation of your look and should represent you as you want to be seen by casting directors. If you are young and attractive, either male or female, you want this shot to show your face, hair and expression to the best of your ability. Consider it a "beauty shot". Make sure your eyes are visible and convey an expression of personality. There should not be a blankness to your expression, casting directors want to see a face that is alive with personality. Yet it doesn't have to be all smiles. If you see yourself more as a brooding or dramatic type, emphasize that. With older actors, you do want your age to show so that you will be considered for roles that are appropriate for you and your look. Use light makeup and make sure your hair looks the best it can. Some airbrushing can be used, but don't take years off your age or remove visible imperfections like significant scars or bags under the eyes. You want the photo to portray the person who will walk in the door if they call you in for an interview.
The Composite Side
Here is where you can show some range. If you see yourself as a very commercial actor, show yourself in settings that portray the kind of characters you can play. For example, if you are a grey-haired, serious-looking male or female, consider a shot with glasses, lab coat and a stethoscope in a doctor's role. Or if you're an action kind of character, sit on a motorcycle or bike. If you are someone whose face shows a lot of wear and tear and who could play gangsters, villains or gang members, show yourself in those kinds of poses. If you are just starting out and could play a range of characters that are "pretty" or "handsome" show one shot with glasses, one in action (e.g., running or dancing) and one that shows your full body, in bathing suits if you have the body for it. Many of these shots can be done outdoors.
Children's Headshots
This is a tricky area. Above all, do not use photos that are more appropriate for pageant work when dealing with little girls. Most all casting directors hate pageant-style photos with excessive makeup and hair on children. The main headshot should be a full face photo and show the child's natural charisma. If your child is an offbeat look, don't be afraid to play that up. Not everyone wants a little precious prince or princess. If your child has teeth missing, play that up. Put a baseball cap on them and have them smile proudly. Casting directors are always looking for every type of child. For the composite on the back, again, show your child in a set of three or four set-ups that show range. If you absolutely must include a pageant shot, you can put one on this side. Use costumes like Girl Scout uniforms or baseball uniforms, too.
