When writing a new post and inserting an image in it, it seems that there is no responsivness for this image (the image doesn’t resize when viewed on a mobile phone)…or am I missing something ?
If a theme doesn’t do what you need, you can talk to the author of the theme to request it (or make a PR), or, you can override the theme templates to add it yourself. Have you examined the theme’s template or shortcode files to see how it works and why it’s not responsive?