Follow these steps to add a poster to a playlist:
Step 1:
Enter in the Content section and open the Posters tab.

Step 2:
Click on the name of the poster.

How to add a poster to a playlist

And then on Add content to a playlist.

How to add a poster to a playlist

Step 3:
Select the playlist to which you want to add the poster and press Add.

How to add a poster to a playlist