I can't spend hours integrating custom website design right now, sorry ... That is just a png icon. The rounded icons in the footer, are FONT (vector) icons, styled and designed meticulously from CSS, which took time. You can't just slap in a PNG there and expect it to look nice.
You can add that icon in the footer like this:
<a href="LINK"><img src=LINK_TO_THAT_ICON /></a>
As you will see, it won't look nice though, unless you hire a web designer to do it properly perhaps ...