You can embed a graphical image in the logout popup window.
This image appears in the window in addition to the text that is displayed. The image must be TFTPed to the switch in the same manner as the custom login image, and must have the filename netlogin_logout_image.jpg or netlogin_logout_image.gif (depending on whether the image is JPEG or GIF). If no such image is present on the switch, then the logout popup contains only text.