The title of your media library, as well as the poster image/trailer, which can be seen in the background of your media library, will be taken from a selected poster VOD, which you can select in the editing interface of your media library under "POSTER VOD". Alternatively, you can simply select "Show first VOD" here so that viewers always get the first video teased.
In the tab "Poster" you can choose whether you want to see a fixed poster image or a moving trailer. Click "Show current poster VOD" to go to the videos editing interface and apply changes directly.
The best way is to create a VOD and give it the title and description that you want your media library to carry, and the poster image/trailer that you want it to adorn. Title, description and poster image/trailer can be set and changed in the videodetails.
When you hit "Save Library" the changes will be visible immediately in your library.