Camera systems vary substantially in size and weight. The lightest auto-advancing film, digital or video cameras may only weigh a few ounces and can be carried in most smaller .20 and .40 sized models with 400-600 square inches of wing area. Heavier 35 mm film and digital cameras may weigh up to a pound or more, and will need a correspondingly larger plane, with 800+ square inches of wing area ideally desired, to reasonably support the increased wing loading.
Although most sport planes will safely accept the addition of a camera, it is the larger trainer-type models which have sufficient wing area to carry the camera aloft with a minimal alteration in flight behavior and performance. Adding the camera can be as simple as strapping it to the side of the fuselage with strong rubber bands, looking along the wing. Alternatively, one can mount it underneath the fuselage pointing down - or mounting it to the top of a high-winged plane or sailplane, looking to the side. It is often possible to mount the camera looking straight ahead, but depending upon camera model, it can induce substantial drag and badly alter the flight behavior of the plane.
How you mount the camera is important too. Vibration really ruins the in-flight photos you will take - so it is necessary to use balanced props, rubber engine mounts, and insulate the camera from as much engine vibration as possible. The insulation also makes good crash protection for the camera.
As such, electrically-powered planes make excellent photography platforms, since there is little vibration under power and the motor can be stopped completely to eliminate vibration entirely. If you fly glow-powered airplanes (slimers...) and are comfortable with dead stick landings (K&B owners? Just kidding...), consider shutting the engine off before taking your pictures. The difference is noticeable!
Figure 1: You can mount the camera inside, if you have enough room
(ex. Lazy Bee, Sig Senior). At worst, you can strap it to the outside
of the plane (ex. sailplane, small trainer).