If you enable the One page per file option in Document properties 1 and turn on Autosave, and choose to autosave as a JPEG, there are two issues.
1. The resulting filename does not append a page number, but instead includes what looks like the printf format specifier for a decimal number, %d.
2. Because of issue 1, if there are multiple pages, you still only get one file, representing one of the pages.
Steps to reproduce:
1. Check One page per file in Document properties 1
2. In Auto-save: check Auto-save and set the Autosave format to JPEG
3. Set Filename to img_
4. Check Use this directory for auto-save and set your output directory
5. Print something to the PDFCreator printer
Resulting file example: img_20100630110546%d.jpg