In earlier versions of Photoshop, I would use the Web Photo Gallery’s numerous image sizing options to batch size images, more often than I would use it to create a photo gallery. Whatever it’s uses, it was omitted from CS4, though the plugin is still available. (There is an image processing feature in Bridge, which I have yet to test).
To restore the Web Photo Gallery plugin to Photoshop CS4, download the plugin package here:
For a Mac:
http://www.adobe.com/support/downloads/detail.jsp?ftpID=4047
For a PC:
http://www.adobe.com/support/downloads/detail.jsp?ftpID=4048
Then follow these instructions, which are way easier to understand than the ones supplied in the plugin package:
http://www.gieson.com/Library/projects/utilities/photoshop/index.html
If everything is done correctly, in Photoshop, you should be able to navigate to File > Automate > Web Photo Gallery.