Change image every scroll

Change image every scroll

Hi guys,

I am trying to implement a script which changes images "every time" user scrolls down.

It means I have 40 images (for example) and I wanna do the following process:
1) first image showed;
2) user scrolls once and the image switches to another one;
3) user scrolls again and the image switches to another one;
4) user scrolls again and the image switches to another one;
...
40) user scrolls last time and the image switches to the last one.

I suppose to use the "scroll" function, but I am not sure if it perfectly suits my need.

Do you have any suggestion?

Thanks in advance.