How do I output one node as XML
I would recommend that you use views as your source for the list of jpegs to output your XML file from. Using Views for example, I was able to make list of event images and make a custom XML file used to display the images in the Cub3r Flash plugin (I thought it was a bad idea but the client really wanted a Flash slideshow). I used the Views Bonus Pack export module, that gave me a style 'XML file'. I had only the image field, that was set to output the full image URL, the label I set it to was used as the custom tag. I then did an override of the XML template that came with the module and customized it to my needs (adding config parameters to cub3r).

Hopefully this will help you out. I also saw that the export module now has a fork that uses the batch API. Make sure you also checkout the Views data source module.