need help for comparing 2 pictures...

Question

now i hava to write a program to compare 2 pic. example, i use webcam, if someone goes to the area of webcam, the computer will capture this picture, that mean the webcam is fixed and if the man moving around, the pic will change and will be an other picture. so i have compare 2 picture. if it has some changes, the computer will capture the picture that changed. someone don't understand, i am sorry, because i am a new English learner. if someone understand my idea, please help me. if you have more time, please sent to me at: [email protected] thanks.