Assing a specific icon for each category

Hello,

I´m looking for a way to assing a specific icon for each categories, so when in the single mode, there´s the title and a category thumb/icon beside. It´s so silly but I can´t find a way to do that.

The thumb/icon are specific and the same for each category. e.g
Food category : a food related thumb/icon
Music category: a music related thumb/icon

I don´t want use plugins if possible.