socialforge/app/views/school/upload_logo.html.erb

<script>
function showPreview(source) {
    var file = source.files[0];
    if(window.FileReader) {
        var fr = new FileReader();
        fr.onloadend = function(e) {
            document.getElementById("avatar_image").src = e.target.result;
        };
        fr.readAsDataURL(file);
    }
}
</script>

<%= form_tag(upload_school_path(@school.id),method: "post",  multipart: true) do %>
	<%#= text_field_tag 'school'%>
    <div style="margin: 20px;">
      <input type="hidden" value="<%= @school_name%>" name="school_name">
      <%= image_tag(@school.logo_link, id: "avatar_image", :class=>"school_avatar")%>
      <a type="button" onclick="$('#file').click();" style="margin: 90px 0 0 10px;float: left;padding: 2px 5px;border: 1px solid #eaeaea;cursor: pointer;text-decoration: none;width: 55px;">上传图片</a>
      <%= file_field_tag 'logo',:style => "display:none;", :id => "file", :onchange => "showPreview(this)"%>
      <div style="clear: both;"></div>
      <div style="margin-top: 10px;">
        <%= submit_tag('上传') %>
        <%= submit_tag('取消') %>
      </div>
    </div>
<% end %>