Those "icon" text fields are not for entering urls or filenames, which is why your icon is not showing what you expect. From the ui_switch node's info:
The On/Off Color and On/Off Icon are optional fields. If they are all present, the default toggle switch will be replaced with the relevant icons and their respective colors.
The On/Off Icon field can be either Material Design icons (e.g. 'check', 'close') or Font Awesome icons (e.g. 'fa-fire').
If you need to show some custom icon images, I think you will have to use a ui_template node -- but then you will have to build the interactivity (to send/recv msg data) yourself.
