Updating scaffolding form field

This may be a basic question, but i have an "image" field in a "news" table, and have a form that uploads the image. Following the upload, the "new" action is called in the rails scaffolding so i can add the rest of the news to db entry. I however want the "image" field to contain the image "id" that was just created in my image table. I send the value to the new action, but cannot get the field to default to this value. As i am fairly unfamiliar with where the scaffolding is storing the information for this form I am having trouble figuring this out. I am unable to find the _form teplate as is described in pickaxe as a partial. Here is the code for the view. The second form is the form I am using to upload the image. So my question is how do i get the image field to use a default value that was passed to the view rather than what is stored as default in my db.

<h1>New news</h1>

<%= start_form_tag :action => 'create' %>   <%= render :partial => 'form'%>   <%= submit_tag "Create" %>

<%= end_form_tag %>

<%= form_tag({:action=>'save_picture'}, :multipart => true)%> <input type="file" name="picture_file" /> <input type="submit" name="Upload" /> </form>

<%= link_to 'Back', :action => 'list' %>