I was having trouble testing images uploads with the Cucumber && Webrat && Selenium setup. This was basically because I was not calling the file path correctly. To prevent this in future, I wrote a helper to extend the Cucumber World object and explicitly

images.rb

module ImageHelpers
  def image_path(size)
    case size
    when "thumbnail"
      File.expand_path(File.join(File.dirname(__FILE__), "..", "support", "fixtures", "adam_mint.jpg"))
      # NOTE: I created a sub dir of "support" called "fixtures" in which to place test files.
    else
      raise "don't know where to find a #{size} image"
    end
  end
end

World(ImageHelpers)

upload_image.feature

Scenario: A Member uploads a photo
    Given I am signed in as the Member "tambourine"
    And I am on my profile edit page
    When I follow "Upload a photo"
    And I attach a "thumbnail" image to "image_source_file"
    And I press "Upload photo"
    Then I should see "This is a caption"

webrat_steps.rb

# added the following step to explicitly handel images 

When /^I attach a "([^\"]*)" image to "([^\"]*)"$/ do |size, field|
  attach_file(field, image_path(size))
end