i believe that converting should be done outside of povray and their are
many programs that you can use to do it but one suggestion is that you
shouldn't try to change the size of the pic outside of povray because
when you do the program stretches the pic and doesn't render it so it
will destroy your pic.
I provide links to a few graphics programs that i prefer:
www.adobe.com
www.jasc.com/pspdl.html