So this is the CSS that would swap out the "plane" fontawesome icon with your whaletail. Obviously when you add the othe "social" icons use an icon that you are happy to swap out e.g. "cog" and chang ethe CSS accordingly. You'll need to adjust your image a little so there is no white space around the image and therefore it fill the character space up.
.fa-plane:before {
content: "_";
background-image: url("/images/whales.png");
background-size: 100% 100%;
background-repeat: no-repeat;
color: transparent;
}
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.