Hi Elisabetta,
You cannot really do anything.
I mean, you can either use small (scaled) images and have them with an awful quality when opened in a Modal (which is quite unacceptable in my opinion) or you can simply forget about the "Serve Scaled Images" recommendation (which I strongly advice you to).
We do not do anything with our demos (apart from optimizing them with JCH Optimize). If you check them with GTMetrix you will see that we also serve the original images (not a scaled ones).
I just cannot be bothered by this recommendation. For me this is just one big nonsense.
Yes, the bigger image will always be bigger in size than the smaller version of the same image. But if you optimize your images with tools like TinyJPG and TInyPNG, the bigger image will be 2, 3KB bigger in size.
My sincere advice is to simply forget about the "Serve Scaled Images" thing, honestly.
You can also read my opinion about Website Optimization in
this discussion.