The site you're importing most likely used this script for lazy-loading images: afarkas.github.io/lazysizes/index.html

As such, images are referenced with a different attribute, instead of the "classic" src attribute.

So either install lazysizes.js (it's a good script by the way, but it won't beat the native loading="lazy" attribute in performance) or perform a database search/replace for data-src/src.

Instructions are available here: afarkas.github.io/lazysizes/index.html

(hint: it's usually at the bottom of the document - just use a script tag)