By default my gmail always displays images.
I installed MailPoet3 on a staging site with similar settings (same theme, same php, many identical plugins) and everything works properly: when I send a preview of a newsletter and open it on gmail it correctly displays the images.
The newsletter also fails to display the social icons provided by MailPoet (png). But some images do.
I tried to replace images that are not displayed with jpg instead of png, but nothing changes.
Unfortunately now I can’t create an administrator account for you to access the site where I have the problem because, due to privacy protection, I have to ask the site owner for authorization (EU GDPR).
The last time I prepared and sent a newsletter from this site on 4 January 2019 (www.prodottivalserena.com) everything worked properly.
But then there were several updates and some new plugins.
But even if the new plugins are deactivated, nothing changes.