1. Use this code...
.popover img {
max-width: none;
}
* Gantry does have some CSS that stops an img exceeding the width of it's container - but this popover code that you're using want to exceed that so I just negated it with CSS.
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
That will allow the image to size to the maximum it has been uploaded at.
2. In the novagem site you had not loaded bootstrap CSS so I manually added that on the page settings to make it work. Bootstrap is loaded by Joomla when told to. But if you don't tell it then it won't loaded - so as one of the site was working I can only assume that something else on the page is asking Joomla to load it.
3. You may find this article that I wrote interesting since it represents a more modern and easier way that you're using for Gantry 5.
https://rockettheme.com/forum/gantry5-for-joomla/284337-how-to-make-a-timed-or-instant-popup-using-the-popupmodule-particle
Regards, Mark.