This fantastic image of Mars was newly released by HiRISE and featured on Discovery News:

In this example, dunes within a Mars crater are detailed (to a resolution of 50 cm/pixel). The image was taken when the southern hemisphere was in the depths of winter in November 2006. The ripples in the dunes are caused by winds shaping the lose dust and sand. The bright areas are either water or carbon dioxide ice frosting the east-facing slopes of the dunes (in the shade from the sun). The darker areas are where sunlight has heated the surface, melting the ice.

If you want it bigger, go here, and down on the right-hand side, pick a wallpaper size to download. (The site, for having so much info, isn’t that intuitive and any link that reads like “This is a larger version of the image you’re looking at” seldom is, I’ve found. What I want is a larger version of the image seen on that page, and that isn’t an easy thing to find.)