The url seems valid. Go to the icon again, double click it to open it for editing and then look at the url produced in preview, that is the url you should use. Sometimes, probably depending on how you add the icon, it is accessible under http://localhost:8080/magnoliaAuthor/resources/templating-kit/themes/myTheme/img/icons/icons/binary.png
instead of under the URI you tried.
HTH, Jan