If the images are purely decorative, you should put them in using the CSS background-image property - the benefits of doing so include the ability to quickly change them by only editing your CSS file, and also they will load last, so your page can render without waiting to download all the images.
However, if the images are important to the message of the page, use the <img> tag and stick them in the code as normal.
Not sure you understood me, and looking at your link I would have said that the images are not important to the message.
By that, I meant if you had a page that said: "Here's a photo of my brother bungee jumping", you would have an <img> element, with alt and longdesc attributes.
Your images, on the other hand, are just illustrative of the sections they relate to - the overall message of the page would be the same if they weren't there, they just augment it - so I would say put them in the CSS.