Batch photo manipulation via free software tools?

I have a need for batch-processing pictures. My requirements are fairly simple:

• Resize the image to fit Facebook's preferred 960 pixel box.
• Insert Copyright, Byline and Bylinetitle into the EXIF data.
• Optionally, paste my watermark onto a predefined corner of the image.
• Optionally, adjust the white balance.
• Rename the file according to a specific syntax.
• Save the result to a predefined folder.

Until recently, I was using Phatch to perform all of this. Unfortunately, it cannot edit the EXIF data of my current Lumix camera, whose JPEG it claims to be MPO. I am thus forced to look for other options. Ideally, I would do this via a script inside gThumb (which is my main photo editing software), but I cannot seem to find adequate documentation on how to achieve this.

I am thus very interested in hearing about other options to achieve the same result. Ideas, anyone?