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Logo or other images missing on retina displays (iphones, macbook pros, other HD/HiDPI devices)

You need to specify an HD version of the image in the theme options. Most importantly, this must be named correctly. It needs to be named with the same name as the non-HD version of the image but it should have @2x right before the file extension, like this:

Original image name: image.png

HD image name: image@2x.png

If you name it something else it will not display on retina display devices.