Try http://irfanview.com/ to batch-convert all of your pictures. It's free and works fine. I've no idea of how to automatically add all those shrinked pictures to your slides, but I wouldn't try to do it in perl. Alternatively I would try it in perl but without using Powerpoint. Maybe http://www.meyerweb.com/eric/tools/s5/ is a tool you would like to use for this kind of task, or the perl-based SPIP http://duehl.de/christian/perl/spip.html but it seems that there exists only a german documentation for it until now.