#!/bin/bash s='<img src="Image1.jpg" alt="boat" /><figcaption>boat</figcaption>' sed 's~\(="\)\([^"]*\.jpg"\)~\1https://e...content-available-to-author-only...e.com/\2~g' <<< "$s"
Standard input is empty
<img src="https://e...content-available-to-author-only...e.com/Image1.jpg" alt="boat" /><figcaption>boat</figcaption>