+ validates :ext, :presence => true, :length => {:maximum => 4}, :inclusion => {:in => ['png', 'jpeg', 'gif']}\r
+ validates :width, :presence => true, :numericality => true, :natural_number => true
+ validates :height, :presence => true, :numericality => true, :natural_number => true
+ validates :filesize, :presence => true, :numericality => {:greater_than => 0, :less_than_or_equal_to => 2000000}, :natural_number => true
+ validates :artist_id, :presence => true, :numericality => true, :existence => true