You need to credit assets it used in an editorial sense. It is fully described in the licensing terms how to correctly credit. Posters are no use in an editorial sense and do not need to be credited (you can, however, credit if you want).
Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788 (Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based on reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).
ABAMBO | Hard- and Software Engineer | Photographer