Some general information about badges:
Find more information on the Week 3 EVO Moderator training page
http://evo-training.pbworks.com/w/page/5965201/Week3#Task3ThinkingaboutCertificatesandBadges
And if you've decided to use badges, this message from the G+C could be useful :
Creating logos and badges and don't have room to credit? Try public domain images. You can get these via http://openclipart.org and www.pdclipart.org