Prestashop image regeneration feature is wonderful but can be long to process for more than a few dozen images. The hack below creates a new regenerate button that does not delete existing generated thumbnails and creates thumbnails for new imported images.

HowTO 

  1. open the admin/tabs/adminimages.php file
  2. modify the following code entries by adding the code in bold
public function displayRegenerate()
    {
         global $currentIndex;

        echo '
        <h2 class="space">'.$this->l('Regenerate thumbnails').'</h2>
        '.$this->l('Regenerates thumbnails for all existing product images').'.<br /><br />';
        $this->displayWarning($this->l('Please be patient, as this can take several minutes').'<br />'.$this->l('Be carefull !! Manual generated thumbnails will be erased by automatic generation thumbnails.'));
        echo '
        <form action="'.$currentIndex.'&token='.$this->token.'" method="post">
            <input type="Submit" name="submitRegenerate'.$this->table.'" value="'.$this->l('Regenerate thumbnails').'" class="button space" confirm(\''.addslashes($this->l('Are you sure?')).'\');" />
        </form>';
       
        echo '
        <form action="'.$currentIndex.'&token='.$this->token.'" method="post">
            <input type="Submit" name="submitRegenerateNew'.$this->table.'" value="'.$this->l('Regenerate thumbnails for new images').'" class="button space" you sure?')).'\');" />
        </form>';
    }
 private function _regenerateThumbnails($new_only=false)
    {
        $productsTypes = ImageType::getImagesTypes('products');
        $categoriesTypes = ImageType::getImagesTypes('categories');
        $languages = Language::getLanguages();

        /* Delete products images */
        if (!$new_only)
        {
        $toDel = scandir(_PS_PROD_IMG_DIR_);
        foreach ($toDel AS $d)
            if (ereg('^[0-9]+\-[0-9]+\-(.*)\.jpg$', $d) OR ereg('^([[:lower:]]{2})\-default\-(.*)\.jpg$', $d))
                unlink(_PS_PROD_IMG_DIR_.$d);
        }
        $productsImages = Image::getAllImages();
        /* Regenerate products images */
        $errors = false;
        foreach ($productsImages AS $k => $image)
        {
            if (file_exists(_PS_PROD_IMG_DIR_.$image['id_product'].'-'.$image['id_image'].'.jpg'))
                foreach ($productsTypes AS $k => $imageType)
                {
                    $file['tmp_name'] = _PS_PROD_IMG_DIR_.$image['id_product'].'-'.$image['id_image'].'.jpg';
                    $file['type'] = 'image/jpg';
                    $newFile = _PS_PROD_IMG_DIR_.$image['id_product'].'-'.$image['id_image'].'-'.stripslashes($imageType['name']).'.jpg';
                    if (!$new_only  || !file_exists($newFile))
                            {
                            if (!imageResize($file, $newFile, intval($imageType['width']), intval($imageType['height']))) $errors = true;
                            }
                }
        }
 

Screenshot