Well, he said that he wanted to "code a layout using only one image," which I'm sure means that he wants to have a whole layout (website design) using one image, which is a bad idea. If there is no text on the page, then SEO will be extremely bad, as will be accessability. He also needs to consider the users on a slower connection (which do exsist, contrary to some peoples designing ways) which will probally not want to hang around waiting for a whole image to load. There are some circumstances where those factors wouldn't need to be considered, such as if you were doing a simple one page design to show a few images and captions.
Saying that, he could just be on about using Image Mapping for one image, such as a map of an area with clickable hotspots on his site. If it is then it is prefectly fine to use that, it could have been that he just worded his post incorrectly.