iPhone 6 Plus & iPad Air 2 media query (Retina HD displays). Also works for similar Android devices.

iPhone-6-plus.css

/* iPhone 6 Plus & iPad Air 2 media query (Retina HD displays).
   Also works for similar Android devices. */
/* Use x2 images by default. */
.logo {
  background-image: url(http://hellyeah.is/images/text-logo-ns@x2.png);
  height: 300px;
  width: 100%;
  background-size: contain;
  background-repeat: no-repeat;
  background-position: center;
}

/* Use x3 images for iPhone 6 plus and similar devices */
@media only screen and (-webkit-min-device-pixel-ratio: 3),
  only screen and (min-resolution: 3dppx), /* Default way */
  only screen and (min-resolution: 350dpi) /* dppx fallback */ {
  .logo {
    background-image: url(http://hellyeah.is/images/text-logo-ns@x3.png);
  }
}

Live result on http://hellyeah.is

that image could use some minification love tho (49.6% savings with imageoptim, for example)

@stryju unfortunately, even though it says it's a lossless compression, it lost some colors due to conversion.