Bootstrap Nav Walker Sub Menu Image

Do you want the same image for each menu item?

.menu-item a:before {
   content: url("/path/to/image.png");
   display: inline-block;
}

The menu items should already have an ID on each <li>

li#menu-item-274 a:before {
    content: url("/path/to/another/image.png");
}

Failing that, you can implement your own Bootstrap Navwalker and add whatever classes or images you like.

See: https://developer.wordpress.org/reference/classes/walker/ and https://github.com/wp-bootstrap/wp-bootstrap-navwalker for how to create your own.

Update:
A final suggestion, based on your comment is to use Advanced Custom Fields so that you can add an image field against each menu item. See this link for details: https://www.advancedcustomfields.com/resources/adding-fields-menu-items/