How to allow user to import image file from drive

you can use www function to load files from hard disk. for file system access use "file://path" and for web resources use "http://url" as the string argument of www's constructor. unity can load JPEG and PNG files and for other file types you need to convert them using external libraries that they are available on the net. for file system manipulation and file processing you need to include the System.IO namespace in your code. you don't need that for simple image loading with www but you might need it for more advanced stuff.