you can resize images dynamically in magento using class
Varien_Image
I've attached a piece of code that I've used for one of my modules
public function resizeImage($width='100',$height='null',$imageUrl){
if($height=='null'): /*for square images, setting equal height and width*/
$height=$width;
else:
$height=$height;
endif;
$folder_path=Mage::getBaseDir('media').DS.$this->_folder_name.DS.$this->_cat_id.'x'.$width.'x'.$height;
$imageName = substr(strrchr($imageUrl,"/"),1);
//$imageResized=Mage::getBaseUrl('media').$this->_folder_name.DS.$this->_cat_id.'x'.$width.'x'.$height.DS.$imageName;
$imageResized=$folder_path.DS.$imageName;
if(!is_dir($folder_path)||!file_exists($imageResized)):
if(!is_dir($folder_path)):
$this->createdirectory($folder_path);
endif;
$dirImg = Mage::getBaseDir().str_replace("/",DS,strstr($imageUrl,'/media'));
if (!file_exists($imageResized)&&file_exists($dirImg)) :
$imageObj = new Varien_Image($dirImg);
$imageObj->constrainOnly(TRUE);
$imageObj->keepAspectRatio(TRUE);
$imageObj->keepFrame(FALSE);
$imageObj->resize($width, $height);
$imageObj->save($imageResized);
endif;
$imageUrl=Mage::getBaseUrl('media').$this->_folder_name.DS.$this->_cat_id.'x'.$width.'x'.$height.DS.$imageName;
else:
$imageUrl=Mage::getBaseUrl('media').$this->_folder_name.DS.$this->_cat_id.'x'.$width.'x'.$height.DS.$imageName;
endif;
return $imageUrl;
}
public function createdirectory($folder_path){
$dir= new Varien_Io_File;
$dir->mkdir($folder_path);
}
replace
$this->_folder_name
$this->_cat_id
with your folder names and category ids
your question implies that the images are auto generated, so even if you delete them, they will be generated again.