Hi pfauchard
As that page indicates that it's combination of Font Awesome font icons with class used in each section.
For example, in the footer of above demo site, you can see a heart icon before the 'ABOUT US' title, here is how it's defined:
.zen-icon-heart:before {
content: "\f004";
}
this content icon will be loaded from the Font Awesome Library in template's css file.
If you want to change to your own image, you can replace above content by URL of your image:
.zen-icon-heart:before {
content: url(" ");
}