What is the retina image?
Some newer devices come with "Retina Display" which , thanks to its high pixel density screen, will make content look sharper and more clear. To ensure your logo has a retina-ready version, you can add one to your theme.
A retina logo image is the same as the original logo image, though twice the size. For example, if your logo image is 100x20 px in size, the retina logo image should be 200x40 px.
You can upload your retina logo image by navigating to Appearance > Theme Options > Logo > Logo image retina