We don’t have an automatic solution for this, but with a little CSS hack, you can easily achieve this.
Create your logo image twice the size you want it to display. Then upload your image as your logo image. Then, with some custom CSS, set the image width to half the size. Here is an example custom CSS.
/* If your image is 500px. */ .header-image-wrapper img { width: 250px; }
This will display your image at half the size, but will make it sharper on your iPhone and other retina screens.