There are multiple ways to manage which images are used to display on category pages.
By default images can be imported from either Google Drive, or a single Folder on your local server (wherever Webninja's Software has been installed on your office network"). In the latter case, the folder must be accessible by the Web Ninja Connector Software - just the same as if a network user in your office were on that server/PC and trying to access the files.
PLEASE NOTE: if you're unsure of the location of this folder - please contact Webninja Support to confirm the exact file path or method in use for your category images.
To add an image to a category it needs to be given a unique name based on the category path (Parent, Sub Category, Sub Category.
To find the correct image naming convention used for a specific category you can navigate to the Category in the Web Ninja CMS
Click on the Category that you're wanting to confirm the image code for, then once the category general info has loaded, switch to the Images tab. 
Look at the Image Code field. This is the name you'll need to save your image with + the file extension on the end. (eg burrs_files_wire_brushes.png or .jpg).
Ensure that file is in the folder you are using as the source of the category images, or in the Google Drive folder that's been mapped to the site.