I think this is a task that is handled by an optional CMS, not the static site generator itself, though I agree it would be handy to have a little helper snippet to do that during build.
There are CMS solutions for static sites which will probably handle images, and all kinds of content, easily. Check out Dato[1], Siteleaf[2] or Netlify CMS[3].
Alternatively you can use external tools to prepare your images and simply rely on the standard HTML srcset within your static site generator of choice. I use RetroBatch[4] to prepare my images.
There are CMS solutions for static sites which will probably handle images, and all kinds of content, easily. Check out Dato[1], Siteleaf[2] or Netlify CMS[3].
Alternatively you can use external tools to prepare your images and simply rely on the standard HTML srcset within your static site generator of choice. I use RetroBatch[4] to prepare my images.
[1] https://www.datocms.com/
[2] https://www.siteleaf.com
[3] https://www.netlifycms.org/
[4] https://flyingmeat.com/retrobatch/