A retina logo image is the same as the normal logo image, though twice the size. For example, if one's regular logo image is 100x100 pixels in size, the retina logo image would need to be 200x200 pixels in size.
Additionally, how you name the retina image is very important. It must be named the exact same as the regular logo image, though with @2x added onto the end of the name.
So, if the regular logo image was named "logo.png", the retina logo image would need to be named "logo@2x. png".