[AdamL56]

You can get photoshop to do this for you automatically.

Open up one of the detailed images. Then you have to create an action. To view actions click Window >> Actions. Then create a new action. It should automatically start recording.

Go Image >> Image Size and input 400px. Then do save (or I always do save for web) and choose settings. And then close the image. Click stop on the action tab to stop recording.

To then automate it for all your files click File >> Automate >> Batch. Choose the action you just created and the source folder then choose an output folder and watch photoshop do it's stuff.

Best to test it out first before running a batch on all your files.