I am usting tb megamenu module to get a main menu on my site. Is there a way to add the website's logo as a top level menu item (NOT SUBMENU ITEM)
You are looking for Menu Icons module.
This module allows you to upload an image in the menu-item's configuration form. After uploading the image, the icon data will be added to the menu link's options array.
When a menu link is goes through the theme layer, it will receive an extra class which by default is used to attach the images as a background image to the corresponding menu item. These class names are also compatible with Nice Menu's.