without correct links to your install I can not answer as I can not see your issue Das grosse Tierforum. de - Artgerechte Tierhaltung
Your forum appears to load for me if this is your site which is the issue you reported with that code hack.
Now as far as your gallery header which is this Fotogalerie - Das grosse Tierforum - Foto Galerie - DgT - Galerie
My suggestion would be that with your custom style either in your custom style settings or the header template of your forum any url to your logo be a full url so it can be shown over another program. Relative urls like /images/misc/logo.png etc would not work you must use a full url to the image so it can be found.
I beleive that would be number 5 on the most common issues thread I linked above.