The file name becomes the title on import if no meta data in the image is set for the title.
http://www.zenphoto.org/news/zenphoto-image-metadata-processing
Only if import of that field is enabled of course.
printImageTitle just prints the title that is stored in the Zenphoto install database, it does not read the description. So if it outputs the description that has somehow been set as the title.
Of course you can print hte filename manually instead by, well, printing the file name ;-)
Btw, if you refer to the site in your profile, be aware that Zenphoto 1.3 is quite outdated ;-)