If you wish to set Featured Images for Categories as you've seen on a few of the Foxiz Theme demos, you can simply open the Categories Page on your site, Dashboard > Posts > Categories. Hover on the Category that you want to set it's featured image and then click on Edit, scroll down the page to the Individual Category Settings Panel then look through the options, you should find the Featured Image Option and then you should click on the 'Choose Image' or 'Select Image' button, then pick your image and save changes.