same thumbnail for a whole category/ first post thumbnail

Hello :]

I am having this problem where i have a site that is category based, so i have lots of categories. inside each archive theres the category description, and there is also supposed to be a poster that is being grabbed from the posts that are in the category.

The thing is, the poster wont show up unless the thumbnail is on the latest post in the category, but the problem is i dont want to set the same thumbnail for each and every post in the category, because some categories have 200 posts in them, so i was thinking that maybe a better solution is for the whole category to have the same thumbnail; that each post grabs the thumbnail from the one before her. OR to set a thumbnail for the first post, and the category will show the thumbnail from the first post only (seems more do-able).

I have tried everything i know, but i just cant get it to set the first post’s thumbnail, it either shows the last post’s thumbnail or doesnt show a thumbnail at all.

Please help :[