Personal – Theme btn images

All of the php, css and theme building of my website https://www.luckyfishgames.com was down outsourced (I have NO idea about any of it). I want to add to a side bar a social media link to google plus. I have figured out how to add the link (finding and copy pasting, adjusting existing code) but have no idea how to add an icon. the code refers to a btn png which I assume is the images used for the bar, but where are these images saved? Are they universal? do they come from some where? I have clicked everything possible (without destroying all the code) to find it. I am at a loss.
Please help!