Simple script that can be added to a Help Center using the Humble Squid Theme. Will give category boxes a background images specified in the array of links to substitute.

humble_squid_cat_images.js

//Begin Humble Squid Category Image Boxes JS Snippet
cat_list = $("ul.category-list li"); // jQuery object containing the category list elements
link_list = $("ul.category-list li a"); // jQuery object containing the category list link elements

// hardcoded array of image assets for use with categories
image_links = [
    "//p1.zdassets.com/hc/theme_assets/link/to/images/for/categories.jpg",//replace with actual image links from your Help Center's assets
    "//p1.zdassets.com/hc/theme_assets/link/to/images/for/categories.jpg",
    "//p1.zdassets.com/hc/theme_assets/link/to/images/for/categories.jpg",
    "//p1.zdassets.com/hc/theme_assets/link/to/images/for/categories.jpg",
    "//p1.zdassets.com/hc/theme_assets/link/to/images/for/categories.jpg",
    "//p1.zdassets.com/hc/theme_assets/link/to/images/for/categories.jpg",
    
];

total_cats = $("ul.category-list li").length; //number of categories we need to iterate over

for (var x = 0; x < total_cats; x++) { //loop to add images to category list elements then strip links of borders and text
    var a_cat = cat_list[x];
    $(a_cat).css("background-image", "url(\"" + image_links[x] + "\")"); //addes background images
    var a_cat_link = link_list[x];
    $(a_cat_link).text(""); //strips link text
    $(a_cat_link).css({
        'border': '0px',
        'width': '100%',
        'height': '100%',
        'margin': '0px'
    });
}
//End Humble Squid Category Image Boxes JS Snippet