Thumbscrew 1.0b12
Have at it. Includes GIF support for thumbnails.
I’m changing the whole Web Page output story, and it’s partially underway, but it’s not finished yet. Basically, I’m making it possible for you to do whatever you want (hopefully) with the end product of each batch run. For the curious, look in /tmp after a batch, and you’ll see an XML file describing all the images processed in that run. This file will be passed to a script that you choose (or write!), so you can do anything you want with the results.
[Update: Fixed the sidebar link. Thanks, Lorne]
