How to add an avatar icon to the Author tag under blog posts?

Hi,

I recently came across a template that had a nice little icon next to the author name under each post as well as having a neat speech bubble icon for the comments, see this image

Is there a way to add my own icons to my posts by adding the code? If so what would it be and where would it need to go? or is there maybe a plug in for this feature?

Many thanks!