Quantity and Variety
The more photos your listing has, the more likely it is that your car will sell. If the online listing is free, upload the maximum number of photos the site allows. With sites that charge for listings, it's worth the extra cost to have more photos in the ad. You don't have to spring for the top package, either. Something that gives you about 10 photos should be sufficient.
Take photos of the car at eye level. There's no need to get creative with fancy high and low angles. Start out in front of the car and make your way around it, snapping a photo from every angle. You may not use them all in the ad, but you can sort that out later.
Make sure you get the basic car-selling angles covered. This includes the front, back, side profile and wheels. Turn the wheels left slightly so that you can take a photo of the tire tread. Finally, open up the hood and take a photo of the engine.
Inside the car, make sure you take a photo of the seats, paying close attention to the driver seat, which tends to get more wear. Sit in the back of the car and recline the front seats to take photos of the front section of the interior. You'll want to show the stereo and instrument cluster, the shifter (to show whether the car is an automatic or manual) and the condition of the steering wheel. Be sure to snap a photo of the odometer so that prospective buyers can see that the mileage reading shown is consistent with what you have said in your ad.
If your car has any special selling points or features (for example if it's a convertible), show them. Photograph it with the top both up and down. If it's a big SUV with folding seats, fold them down to show off the storage space.