While it is possible to set the item image using the item's CssClass property (http://www.telerik.com/help/aspnet-ajax/menu-appearance-item-images.html), it would be great to be able to set that directly to the span element with class rmLeftImage.

Otherwise, it's necessary to use JavaScript or a CSS pre-processor like LESS to assign font-based images via classes (LESS would be required only to avoid duplicating CSS).

The same would apply to just about any control that supports item images, e.g. TreeView.