SSL difficulties after WP Multisite migration

Dear friends,
I was really pleased to see the hosting service within the membership and I have decided to transfer all my sites here. When I created new sites, everything worked perfectly so I have decided to migrate my main wordpress MU installation. Originally the installation was without SSL certificate and after the migration I have difficulties with some pictures on some blogs. I can see them in Media library, but they do not show on page. If I open the picture on their own in browser, it says that the site is not safe and the certificate is not valid.
Can you, please give me an advice how to solve this issue?
Thank you very much.
Best regards
Petr Sahula