Hi.
I put the words: 'moodle, overlay and logo' in google and the first page I came to answered your question:
https://moodle.org/mod/forum/discuss.php?d=164795
If you need to ask anymore questions regarding your theme you'll get a better response in the theme forum:
https://moodle.org/mod/forum/view.php?id=46
Cheers, Sam Thing.