I'll put in my perennial plug for Eyebatch by Atalasoft.com. It's a couple years old and they aren't updating it any more, but it's an amazing program and more flexible with resizing, watermarking, converting from one format to another, and renaming than just about anything else out there.

If you need to resize, modify, watermark, and add the Digimarc adult content watermark, then Thumbs Plus is the way to go. We've switched to that from Eyebatch to get the Digimarc support, and it's probably a little faster, though its renaming abilities aren't as good as Eyebatch.