Rails button_to with image and actions

I want to show button with image. I have this code

<%= image_submit_tag "down.png", controller: "posts", action: "votedown", post_id: post.id, topic_id: post.topic_id, class: "xta" %>

Its visible properly but not calling action "votedown"

In my routes I have

post '/votedown', to: 'posts#votedown