How to post a "poster" to VKontakte social network wall

I have a script that can post a text or an image to the VK.com over API. But I can't find a way to create a poaster:

There is no information about posters at the official documentation page.

You have to add the parameter poster_bkg_id = {background_number_for_poster}. This parameter is not described in the documentation (thanks to the author https://github.com/VBIralo/vk-posters).

Backgrounds (poster_bkg_id)

• Gradients

  1, 2, 3, 4, 5, 6, 7, 8, 9, 10

• Artwork

  11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 31, 32

• Emoji Patterns

  21, 22, 23, 24, 25, 26, 27, 28, 29, 30