app/views/tags/_follow.html.erb
<% if current_user.follow?(tag) %>
<%= link_to unfollow_tag_path(tag), method: :post, class: "follow-link", remote: true do %>
<%= image_tag "stocked.png" %><span><%= t "tags.unfollow" %></span>
<% end %>
<% else %>
<%= link_to follow_tag_path(tag), method: :post, class: "follow-link", remote: true do %>
<%= image_tag "unstocked.png" %><span><%= t "tags.follow" %></span>
<% end %>
<% end %>