Thanks for the quick response. Your link wasn't very helpful. I found this one more straightforward & helpful> http://themeid.com/forum/topic/3831/php-files/
This is what I did:
1. I added an /icons/ folder here> public_html/wp-content/themes/responsivechild/icons
2. I downloaded free icons from here> http://dryicons.com/free-icons/preview/socialize-icons-set/
3. I changed the 2 .png files I downloaded to> twitter-icon.png & facebook-icon.png and added them to the /icons/ folder.
4. I refreshed my website and the 2 links with the question marks were replaced with the .png files I added to the /icons/ folder.
Can you please let me know if I did this correctly? Are the free icons I'm using ok or do I need to use some other WordPress approved ones? Again, thanks for your help.