Few months ago i wrote an article to share about the way to show categories images on Prestashop Menu.
Then @Psdesigner and @Damien Belgacig asked me about the way to put subcategories images instead of main category's image. Sorry 2 guys, Disqus didn't tell me about your comments.
Well, as my previous article, if you edit ps_mainmenu template file (ps_mainmenu.tpl) and put the source code as my suggest, all categories images will be showed on Prestashop Menu. In case you only want to allow subcategories images, we need to make some changes.
Edit your subcategories and upload menu's image as my previous article. You can't show the images if you don't have at least one, right?
You need to make sure the main category is the root menu, and the categories are the submenus. I don't have time to play with Prestashop default menu so i don't know can we setup a subcategory as a root menu or not (we have SOO Magic Menu - do you remember? We love it so much and hope people use it everyday :D).
Re-open your-website/themes/classic/modules/ps_mainmenu/ps_mainmenu.tpl file and find the code around line 27.
Visit my page to explain how to play with your Prestashop website : https://www.prestasoo.com/blog/how-to-show-subcategories-images-in-prestashop-s-top-menu.html