In here, you can upload the pictures you want to use in your templates.
You access it by navigating to DOCUMENTS > FILES in the Setup-module as shown.
To upload an image, simply click on the "Add file"-button:
To reference the images you have uploaded, you just assume that the files are in the same folder as the template you are using it in.
Please note, these errors can depend on your browser setup.
If this problem persists, please contact our support.