ALT tag to logo image in branding, header

Hello. I just wondered if you can give me a hint to print an ALT tag in logo branding;

So far I think that I can add_filter in my child > functions.php

Something like this:

function add_alt_tag($nirvana_siteheader) {

....The thing to do
}
add_filter('cryout_branding_hook','add_alt_tag');

I really dont know how to do this, since my knowledge of hooks is very, very poor

I hope you can help me!
Thanks in advanced