Just download the zip-folder (look for the download button on the right). You will find several files there with different resolution. Just pick up the one that fits your screen and set it as a wallpaper.
Thank you! Well, I've just saved the picture of iMac from Apple Website, erased the white background, and pasted the wall on the background and on the screen. Than I used free-transform to make the last one fit the screen's size and perspective. Regarding the reflection, I've created a new layer, picked up the place where I wanted reflection to be with lasso tool, and used gradient tool (from white to transparent). Like this! I did it all in PhotoShop.