Responsive images have been around since SiteVision 3, but from version 4.1, SiteVision delivers a scaled-down version of the image that is tailored to the visitor's device. This improves performance for both the visitor and the server.
Optimised images can be found in the modules Image, Random image, News, Blog, Link List and Slideshow. The website must be constructed using responsive templates. The function is based on image not only getting "src" in the code, but also "srcset" which contains the scaled-down version. Optimised images can be accessed via the public API (ImageRenderer).
If you have permission to "Manage functions for developers", there is an additional checkbox in the settings for the Image module. This checkbox allows you to disable the optimisation.