Thanks to Jacob’s hint I got the solution. This one is working.
Header set Content-Security-Policy "base-uri 'self'; default-src 'self'; font-src 'self' data: https://fonts.gstatic.com; frame-src https://www.google.com https://www.youtube.com; img-src data: 'self' https://secure.gravatar.com; script-src 'self' 'unsafe-inline' ; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; object-src 'self'; form-action 'self'; frame-ancestors 'self';"
The difference is the data: in the ìmg-src section.