Hi @Dominik Weber,
Hope you are doing well.
In order to Address your Microsoft Bookings related query "Is it possible to personalize the banner image in Bookings with me? Or will this setting still exist at some point?", it's suggested that you could post your case in dedicated forum for Microsoft Bookings for professional advice.
Hope this helps.
If the answer is helpful, please click Accept Answer and kindly upvote it. If you have any further questions about this answer, please click Comment.