For all Joomla 3+ templates built using the Zen Grid Framework v4 (any theme after October 2014) please refer to the Zen Grid Framework v4 documentation.
Social icons are part of ZenGrid Framework v2. Inside template setting under Tools->Social Icons you can define link to social services of your choice.
All images you can select inside dropdown are loaded from folder withing your template: Joomla root/templates/YOUR TEMPLATE/images/icons.
Once you upload image in this folder it will be inside image dropdown for social icons next time you open administration.
Social links are links to social services of your choice. Example of such a link: http://www.twitter.com/joomlabamboo
Social icons code is generated in this file: plugins/system/zengridframework/zengridframework/layout/socialicons.php (for Joomla 1.7/2.5). If you don't want to loose your design/layout/function changes you need to copy this file inside your current template. How to copy: copy socialicons.php file into your template folder/layout. Now you are safe and your changes will not be reverted after framework update.