The <img> tag is for use inside a tag, such as <summary>, <remarks>, or <returns>, and lets you add an image or a class diagram to the text. Image or diagram will appear in resulting documentation on the place of <img> tag, exactly like this logo.
All image files (except class diagrams) should be located in your folder for external files.
To insert class diagram, you specify its .cd file with path relative to the project root path. The best way is to use comment editor.
<img src="image_file_name|class_diagram_file_name" />