Exporting images to XML

I'm writing an application that holds a signature image in a SQLCE
database. I need to export this image to an XML file. I have tried this (in
C#) with DataSet.WriteXML(filename) and it works fine; the resulting image
seems to be compressed (a little under half the original size) and is
human-readable (no weird unprintable characters). However, I am parsing all
the records and exporting only the fields I need.

I have two questions:

1) Is there a way to tell WriteXML() which fields I want to export?

2) How can I compress/write the image data the same way as WriteXML()?

Thanks,
Kevin