In your shoes, I think I would also be annoyed by the time I spent on my poster. For whatever reason, they didn't choose yours, but you made it while you were under the impression that the time you were spending was going to bear fruit. After all that thought and effort, you weren't rewarded (or thanked?). I'd write to the person who requested that you create the poster and I'd say something like
"While I understand that mine might not have accomplished the goal you were looking for and so you chose to go with something else, what I don't appreciate is that I spent a lot of time putting forth effort and then without warning, my work was not used. It doesn't encourage me to continue to volunteer making posters because I feel like after a lot of time and effort, my next work might not be used either. In the future please X, Y, Z" and explain that you would have liked a private message warning you that your work would not be used or whatever you would like in the future.
I feel like they should have indicated that they appreciated your time and effort, and they should have warned you that they weren't going to use your work, even if they don't want to tell you the reason why not.