I am not sure if demo on your site doesn't work or I am missing some step. 

I want to see how the plugin set prices for different sizes of images, in my case due to the different size of the print. 

Also, I was wondering is it possible to set different pricing rules for different products?

Do you maybe have or plan to create video that shows how this Pricing AddOn really works?
Or do you have some document to show us what to do? Maybe we are doing something wrong in setting the prices. 

My friends also tried to see how it's working but they came to the same impression that it's not working as it's said. 

I tried everything, but it doesn work. I think maybe the button "apply pricing rules" doesnt respond when I click. There is no change after that, no matter what I do. Price is changing only when I add more elements.

I would like to see how the price is changing when it depends on the image size rule.

When I add new image the blue margin that defines image size is not there. I can resize the image all over the tshirt, and there is no crop at all, and the price is always the same.