Two popular programs included with OS X are Photo Booth and iChat, both of which allow you to use a camera attached to the system to capture and send pictures and videos of yourself to friends and colleagues.
Both of these programs have a video effects section, where you can apply filters to stretch and warp your face into odd shapes, or have hearts or birds revolve around your head for a "lovestruck" or "dizzy" effect. While these features should work just fine, there are situations in which the effect either cuts in and out or does not work at all.
The effects in iChat and Photo Booth use facial recognition technology so when you move within the frame of the camera the effect moves with your face. In order to work properly, the effect needs to be able to detect your face, so first make sure you have proper lighting so your facial features are clear. Even though the facial recognition works fairly well under low light, it may have trouble if the lighting is too low. In addition to making sure light is at the proper levels, try taking off any headgear like hats or glasses that might be covering your face, especially your eyes.
Beyond these situations, on some systems the video effects may not load at all. When you open the effects preview grid, all the effect panes look like a standard view and clicking them has no effect on your image. This happens because the facial recognition components of the software are written in 64-bit code and therefore require the program to be running in 64-bit mode on a 64-bit-capable machine. If your Mac does not contain a 64-bit processor then you will not be able to run these visual effects in iChat or Photo Booth.
If your Mac does have a 64-bit processor and the effects are still not working in either iChat or Photo Booth, then get information on the program in the Finder and make sure the option for them to run in 32-bit mode is unchecked. While the programs are fully 64-bit-capable, Apple does include the 32-bit code for them for compatibility, even in its latest operating system (OS X Lion) which will only run on 64-bit-capable machines. If you were running iChat or Photo Booth in forced 32-bit mode on Snow Leopard before upgrading, then the program may stay with this setting when you upgrade, with the result that the video effects stop working.