The way the retina works in the Total theme is that any theme function (featured images, Image element, Image Grid, Post Cards, etc) will automatically generate a @2x version of a displayed image if the uploaded image is large enough to generate a version that is at least twice as big as the defined crop size.
For Example: If you go to Theme Panel > Image Sizes you can see by default there isn’t any cropping defined. If you set your blog entries to crop at 500×500 then the theme will create a cropped image at the defined size (if possible) as well as try and create a 500×500@2x version that is 1000×1000 pixels.
If a retina image is generated it is added to the image HTML tag via an srcset tag, this way the retina image is only loaded when needed, the theme does not use javascript for retina images thus keeping your site as fast as possible.
What Images Does the theme Not Generate Retina Versions for?
Since the retina functionality in the Total theme is built-in and requires images to pass through the theme’s methods, any image displayed by a 3rd party plugin or child theme code that isn’t using native theme functions will be excluded from the retina image generation. So for example, the WPBakery “Single Image” element will not render a retina version, however, the Total “Image” element will.