Welcome to Demiplane! I’ll go ahead and answer these in order.
At the moment there’s not a way to hide unowned content in the VtM Character Builder.
Can I somehow change the game title picture to freely chosen one? The premade library has nothing for VtM?
Not at the moment. I have an open request to add the ability to do this and I’ll make sure to add your voice to that to let the team know it’s something that folks are looking for.
I made session notes into session and shared them but my players see just empty note, is this a bug?
This definitely sounds like a bug. Just in case though make sure that you clicked the share button and that there’s a checkmark next to the name of each player you’re trying to share a journal with. If they still aren’t able to see the journals after that reach out to our support team for more help.