In order to have a custom icon in a dialog window, you must create or find one on the web. This image must be converted to an ICNS file (icon.icns). You can download different converters online to export a PNG as an ICNS file.
In order to have an icon in your script, you must have a Script Bundle. This type of script has its own file library to save and reference your resources. For icons in Plugin Assistant, please refer to the Plugin Assistant Developer Center