resize_image.rb

class ResizeImageError < StandardError;end  
IMAGE_SIZE = "720x720>"

def resize_image
    result_path = File.dirname(self.file_path) + "/#{self.user_id}_#{Time.now.to_i}_#{(rand * 10000000).to_i}.jpg"
    cmd = %(convert -strip "#{self.file_path}" -resize "#{IMAGE_SIZE}" "#{result_path}")
    output = `#{cmd}`
    unless $?.exitstatus == 0
      raise ResizeImageError, "Error while resize image #{cmd}"
    end     
    self.file_path = result_path
    self.file_size = File.size(result_path)
 end