Use this snippet to add a watermark to an image, after the image is uploaded:

   1  class Image
   2    .
   3    has_attachment ...
   4    .
   5    .
   6    after_attachment_saved do |record|    
   7      # Don't add watermarks to thumbnails
   8      if record.thumbnail.nil?
   9        full_path = File.join(RAILS_ROOT, 'public/', record.public_filename)
  10        
  11        img = MiniMagick::Image.from_file(full_path)
  12        
  13        width = img[:width]
  14        height = img[:height]
  15        
  16        if width > 150 && height > 150
  17          img.combine_options do |c|
  18            c.gravity 'SouthWest'
  19            # This is RAILS_ROOT/images/watermark.gif
  20            c.draw "image Over 0,0 0,0 \"images/watermark.gif\""
  21          end
  22  
  23          img.write(full_path)
  24          
  25        end
  26      end
  27      
  28    end

Note that after_attachment_saved is a callback added by attachment_fu, use after_save if you’re not using attachment_fu.