Replace title with logo in header

Hi everybody!
I know, this thing has probably been asked a thousand times everywhere, but I couldn’t find a solution to my problem, partly because I’m not really an expert, partly because I don’t understand the structure of the theme.

The theme I’m using is Yoko (with changes in a child theme) and here are the relevant files in pastebin:
Yoko’s header.php: http://pastebin.com/zQrrcr6A
Child theme’s header.php: http://pastebin.com/ZMrCp0ka
Yoko’s CSS: http://pastebin.com/rbE5n6F5

It seems that the thing I should change is the header.php file, because there’s nothing relevant in the CSS, but I couldn’t find the code I should tweak.

Thanks for reading (and hopefully for helping!)