Two good sites for free monkey images are Flickr.com and Commons.Wikimedia.org. Flickr alone has a Monkeys and Primates group which contains 18,700 images as of mid 2015.
Wikimedia Commons page divides its main monkey page into New World and Old World genera. New World genera with their own pages include spider monkeys, howling monkeys, tamarins and marmosets. Old World genera with their own pages include vervets, macaques, baboons, langurs and colobus and proboscis monkeys.
Most images at Wikimedia Commons are free to use as long as the photographer is given credit. Photographers at Flickr.com vary in the permissions they give to reproduce images, but they indicate whether all or some rights are reserved.