Change thumbnails url

Hi,
i have a cdn that serves image sizes by url, like:
“https://cdn.domain.com/img1/s64.jpg” (small),
“https://cdn.domain.com/img1/s500.jpg” (medium),
“https://cdn.domain.com/img1/s2000.jpg” (large).

so i want to use the ‘medium’ size as shop/category page product thumbnail, the ‘large’ size for single product page, and the ‘smaller’ size for backend admin area product list.
because the usage of original large image in shop page (like now), cause huge page size and loading time.

any suggestion?

Sorry for my bad english and Thank you.