I need to add an image of each category

The easiest method is to do something similiar to this:

https://stackoverflow.com/questions/6668577/using-before-css-pseudo-element-to-add-image-to-modal

Although I’m not sure how you are displaying your categories, this might also be what you are looking for:

https://en-ca.www.ads-software.com/plugins/categories-images/screenshots/