gistfile1.rb

# model
class GiftIdea
 has_many :images
end

# in the view (say app/views/gift_ideas/show.html.erb)
# say we are rendering html about one GiftIdea object, found in the @gift_idea variable
..
..
<div class="gift_idea">
 <%= link_to @gift_idea.title, @gift_idea.url %>
 <img src="<%= gift_idea_image_url(@gift_idea) %>">
</div>
..
..
# in GiftIdeasHelper (app/helpers/gift_ideas_helper.rb)
def gift_idea_image_url(idea)
  if idea.images.present?
    idea.images.first.url
  else
    "/path/to/generic/image.jpg"
  end
end