Pretty easy actually. YOu need to have all the photos in one folder.
What you can do is creat an action. Open one of the photos from your folder. Go to your action window and hit the create new action icon. Then hit record.
You can then go to the photo and image->resize and enter 72dpi, i think you will probably have both vert and horiz photos? instead of puttin in 700 on one side what you want to do is resize via percent on both the vert and horiz. that way you won't get funny sized photos.. then hit enter..
You can then hit Save As and put in a seperate folder and then close. Now hit stop record on the action menu....
NOw, go to File -> Automate -> Batch. simply choose your new action from the drop down menu and enter the source folder and destination folder and hit go! Then go have a beer while you wait.. 