How to Center the Category Image

Hi,

You've configured the menu item to have 3 columns of categories. With your template, it means that each element has a width of 400px available. The text is centered on that width.
The thumbnails width for the categories is configured to be 300*300px. So a width of 300px.
The best would be to either increase the number of columns or increase the size of the thumbnails for the categories in your menu item via the Joomla menu manager.

You could also center the image or make it stretch with CSS code, but it feels less clean and more complex than what I suggest above.

Hello,

You have to target your relative category image, and define an auto margin (right & left), follow this scheme :

.your_categories_listing .image_container img {
    margin-right: auto;
    margin-left: auto;
    border: 1px solid black;
}

Hello,

Sorry for this return but we aren't sure to understand what you mean with your last message.
Regards

Hello,

Because you have to build your own selector css ( documentation ), and to get your required html class, you have to use your browser inspector tool.
1. Point your element => Right-click
2. Select Inspect (or inspector tool)
3. A new window will be open, allowing you to see your element class AND his container class