Hello,
Can you tell me what is the environment (what kind of application, is it online or offline, who will use it and most important: what is the level of security needed).
I'm afraid this idea won't be much secure. Let me tell you an example: let's imagine the image will be 800x600, and the area that gives you access is 5x5. This results in 160 x 120 different areas, or 19200 possibilities. Using only numbers and lowercase letters, this would be less than on a 3 character password.
If the way this will be done has any chance to let a bot trying every possibility, it will be very easy to crack it.
Maybe this level of security will be enough for your need, maybe not.
We can discuss this a little bit, and then if you wish I can help you implement this or give you advice on how this should be done. My fee is INR 520 /h . More about me you can ask or find on my profile.
Best regards,
Arthur Lopes