thanx Nioclas for the reply, but somehow I got stuck - and I thought that I can get the missing link here. Pls. let me try again:
so far I could narrow down the url, where the png files are take from: e.g.
But there's no path underneath /var/www/ which looks like this and contains the png files (e.g. moodle-logo.png). So, any hint of how this works and I could replace it w/ the logos I'd like to apply for my use case would be more than welcomed.