When I set the reponsive images, the main image is correctly showed (correctly adjusted)
But all small other images are expanded to fill the size of the page.(with bad resolution)

When I not set the responsive images, all the small images are not adjusted. Correct size.
But the main image is reduce regarding the page size but not in the both dimension.

What I need to enter in the responsive images CSS selectors to make responsive only the main image ?

Example of the problem https://www.ornicom.com/fr/produits/communicateur-ip-avec-telco-ipcom.amp.html

Obviously, it's not a problem it's how things work.
For this reason there are 2 configuration parameters that allow to select certain images to be responsive or to exclude certain images to not be responsive.
What you need to enter depends on your images, for example to target the main image of Virtuemart products:

Code:

div.main-image img

To find CSS selectors use an inspector tool such as Firebug, or check the source code directly.

The image is in the correct size !

Thank you for your support