Hi, I'm not sure if I understand you correctly. What is the scenario behind this question? Where do you want to display these icons?
You can specify the document type icon according to https://docs.kentico.com/display/K8/Changing+document+type+icons.
But if it doesn't fit your needs, I guess you need to create a new text field with a predefined URL as default value and untick the "Display field in the editing form" checkbox. I'd recommend to store these images directly in the file system (for better performance).
Is that what you need?