Thank you for writing in! To change the icons or graphics on your menu items, please edit your menu in Appearance > Menus and select your desired icon or add the image URL on there respective fields.
Those fields should be there by default, if you don't see it make sure your PRO and Wordpress are up to date.
Or maybe you have a 3rd party plugin that terrorizing those fields. Please do a testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Please let us know how it goes.