What png file you are trying to change and where do you want to use the icon? If you can send me a screenshot or a link to your site and specify where you want to use the icons, I can suggest a solution. If you just want a car icon image file then here is a link to find icons https://www.iconfinder.com/search/?q=car
You are modifying the wrong file. You will have to modify the “icomoon.svg” file in the “CSS\icon-fonts” folder. You can load that file in the icomoon app ( http://icomoon.io/app/ ) and modify it. You can add new icons from the icomoon recommended icon packs to your app and export the fonts package. Hope that helps. Thanks.
I have attached the icon for the “car”. After uploading the icon to your media library, you can have a custom CSS specified in the Custom CSS tab in the theme options panel for this icon to take effect –