spec => {
   img_1 => {   
     gen_files => {   
           'img_1_thmb_1' => {                                                                          
                   transform_method => \&gen_thumb,                
                   params => [{ w => 100, h => 100 }],                                                             
           }                                                                                                         
           'img_1_thmb_2' => {                                                                                      
                   transform_method => \&gen_thumb, 
                   params => [{ w => 50, h => 50 }], 
           }                                                                                                          
   }                                                                                                                
 },   
},