Writebytes

I'm trying to download a file to disk, but it doesn't seem to be working. I'm not sure what to put for for bytes and length, and if I need to use a URLLoader to load the image first, or if I can write it as long as I have it's path.

filename = "1.jpg";

var myFile:File = File.documentsDirectory.resolvePath(filename);
var myFileStream:FileStream = new FileStream();


myFileStream.open( myFile, FileMode.WRITE );
myFileStream.writeBytes(??, 0, ??);