PDF::Writer：#image で :border オプションを指定

require 'pdf/writer'

pdf = PDF::Writer.new
pdf.image('images/chunkybacon.jpg', { :border => true })
pdf.image('images/chunkybacon.jpg', { :border => { :color => Color::RGB::Blue, :style => PDF::Writer::StrokeStyle.new(20) } })

File.open("2008022100.pdf", "wb") { |f| f.write pdf.render }