You create an action that resizes, then Save AS your file type. Then you run the action in batch mode using File|automate|batch. You can read about these in the helpfile.
One little "trick" is to toggle the dialog of the Save As portion of your action to OFF. And in the Batch Dialog, make sure and Check the box that says Override Save As information. This passes control over filename and location to the batch dialog.
You may not know what I’m talking about yet, but trust me, when you get there, you’ll see.
Peace,
Tony