Why doesn't CDN work with images?

Question

1C-Bitrix

When CDN is enabled, pictures continue to be downloaded from the site (do not precede links with cdn bitrix). What is it?

1. Everything will be fine, it takes time
   
2. Pictures must be inserted by some special method (like setAdditionalCss() for styles)
   
3. Other
   

Somewhere I met such a method that generates markup for inserting images on the site - I did not attach any importance to it. Perhaps it is just for the CDN to work with images?