Kanso contains a script that automatically replaces images with their high-resolution variants on retina and similar displays. This means that you need to upload a separate optimized version for each image, into the same folder as the original one, and name it using Apple’s prescribed high-resolution modifier (@2x): e.g. „firstname.lastname@example.org“ will be regarded as a retina version of „image.jpg“. The images can be of the same resolution, but twice as large, or of the same size, but of twice the resolution (i.e. for a 50dpi, 300×300 image a retina version would have to be 50dpi 600×600 or 100dpi 300×300). This is a recommended solution since it does not overload the pages unnecessarily on lower-resolution displays, trying to load larger images.
You could, of course, simply use large and high-resolution images throughout the website, in which case they would stay crisp on hi-res displays (note that in this case the performance for low-res displays will suffer).
Finally, if you need more fine-tuning (e.g. making only some of the images retina-ready), I would recommend using the WP Retina 2X plugin.
You can find additional information about the hi-res image usage in Kanso on pages 12-13 of the theme manual.