Here are some genealogy sites that mention Reso:
http://boards.ancestry.com/mbexec/board/an/surnames.resohttp://www.geocities.com/mary_lawrie/index3.htm#Reso
The first site implies an Italian origin and a relation to the surname Riso. My reference book lists an Italian surname Risso, which is a Ligurian variant of Ricci, which means "curly-haired".