By default, a folder icon is used for your category. In the category form (when creating or editing a category), you'll find a dropdown for the icon.
We use FontAwesome
for powering the icons, and it's awesome. (See what we did there?) FontAwesome has a fantastic search tool. Once you've found the perfect icon for your category, select it from the dropdown in Fieldhelp, and you'll be on your way. 💅