You can add image to your categories and sub-categories in Step 2 on Product & Categories page. Follow bellow steps to add the image.
- Select the category or sub-category for which you want to add the image.
- Click on the box under the Step 2 of the page.
- A Media library window will open where it will store all the media files which you want to show on your catalog.
- Upload your image in media library by clicking on Upload file button.
- Select the file in your computer and upload.
- After successful upload, click on image in media library to select that image as a category or sub-category image.
- To verify, it will show the name of the image in the box in Step 2.