Create a style in your css that sets the background image for each type of tree node:

remember to change [path_to_your_image_location] with the relevant path on your system.

Create your own custom getIconClass function to override the tree's default:

//custom getIconClass function for the role tree
var iconFunc = function( /*StoreItem*/ item, /*boolean*/ opened ){    
    return item.icon[0]; //this is hacky - should really use the tree store's to get the item's 'icon' value
};

iconFunc is just an example name.

Then, when creating the tree that will show the data, in the constructor params set:

getIconClass: iconFunc,

Then change your json data so that the 'icon' property of the continent item is 'icon: "continent"'

Thats it - but you should probably take note of the hacky comment in this implementation and use the tree store to get the icon property of the selected store item.