Code For Placing Avatar in Header

Hi could anyone help me with a piece of code ( or guidance as to where I could find some) to place an 'Avatar Box' centrally in my header, and which can have images loaded from the Appearance/Customizer?

Have searched in vain!

This theme actually does it very nicely!!!!!!!!!!!!!!!!! :-

https://wordpress.org/themes/accessible-zen/

Thanks in advance.

Regards

John