Mass Resizing Photos in Photoshop

Warning: If you don't do this right, it will resize your original images. So, make sure you have a backup of them before you begin! You have been warned.

1. Put all of the images you want to process into one directory then open any one of them in Photoshop.

2. Go to Window --> Actions to show the palette. At the bottom of the palette, click the small page-turning icon to create a new action. Give it a name and click [OK].

3. At the bottom of this palette, click the small circle button ("Begin Recording"). This initiates a VCR-like functionality that will record all of your actions and store them so that they can be repeated on other images.

4. Go to Image --> Image Size. Here, specify the new image size; you can change the select pop-ups for either the "Pixel Dimension" group or "Document Size" to change the units. Either one of those will resize it correctly. Click [OK] when you're done.

5. Go to File --> Save As and save it to whatever location (you can specify a new folder) with the right format, compression, and output settings.

6. Now go back to the Actions palette and click the small square button ("Stop Recording") to stop and save the recording.

7. Close the currently opened image.

8. You are now ready to automate the process, go to File --> Automate --> Batch. Set these settings:

Action: the action you just created
Source: Folder
Choose...: select the folder where your images to process are
Destination: Save and Close
Override Action "Save As" Command: No

9. Click [OK] and let Photoshop process all your photos. When it finishes, all your images should be resized in a new folder (if you set that).

References:
VBulletin
or for a tutorial with photos:
Amy Williams Design