setting a scale by clicking on excel to determine lowest and high x values and .....

Hello first allow me to apologize if i posted this in the wrong section.

I am in an excell and vba class and have been assigned a final project. We were allowed to pick our own project and i have ended up a little in over my head. I have been looking stuff up for a whole week now and still havent found and substantial information (or atleast easily explained not sure if i stumbleupon answer and didnt think it was helpful). Any way here is my project

I am supposed to upload a picture on excell and by clicking on it point to complete a polygon it should tell you the area. basically a planimeter.

but i cannot figure out for the life me how to be able to click on the picture and enter your highest and lowest x and y values

If what im saying makes sense and anyone has information it would be greatly appreciated

thanks