Originally Posted by JamesK

If you have Photoshop you can easily do it with that. Just go to "Actions" window, make a new directory, press record and go to "Image" -> "Image Size". Now set the width to 390 pixels and stop record.

To run this process on a whole batch, go to "File" -> "Automate" -> "Batch", select the action you just made and you're ready to go.