Graphic Converter converts a slew of image formats. It's $30
shareware from versiontracker.com -- and well worth it.
But a ".pngt" file is the same as a ".mac" file. They're aliases
of MacPaint. See:
http://www.fileformat.info/format/macpaint/egff.htm
I sincerely doubt that MacPaint, if you can find a copy, will run
under Classic. Way too old an app.