Think of how you would laugh naturally in a normal setting. (Meaning not while you are on camera.) You'll notice that first you hear or see something funny, an overall sense of pleasure grips you, your mouth begins to open as your cheeks break into a smile and finally you exhale a bit of air.
Practice the art of how you laugh. Some actors swear by looking in the mirror to see this transformation, while others feel it is "false." Apply your own tenants of acting while you take yourself through the "anatomy of your own laugh."
Listen and respond each and every time you hear the line or see the image. This can be tough to do as you've played the scene for the twentieth time. But it is essential that you truly listen to the other person before you begin your laugh. Otherwise it will strike as false.
Avoid pushing the laughter. For the most part (unless you're in a sketch comedy) laughter is personal, it's intimate. It's an expression of you and your feelings. So if you suddenly feel as if you are forcing the laugh, then you are. Take a deep breath and go through the anatomy of your laugh again to find the "truth."
Make every laugh slightly different. In life, laughter is spontaneous. Therefore, no two laughs are exactly alike. Run the gamut of giggling to guffawing based on what the situation calls for and you'll be laughing naturally on cue in no time.