Looking at the product converted used in the dataflow module Mage_Catalog_Model_Convert_Adapter_Product
it would appear that it can only deal with one product image :(
foreach ($product->getMediaAttributes() as $mediaAttributeCode => $mediaAttribute) {
if (isset($importData[$mediaAttributeCode])) {
$file = trim($importData[$mediaAttributeCode]);
if (!empty($file) && !$mediaGalleryBackendModel->getImage($product, $file)) {
$arrayToMassAdd[] = array('file' => trim($file), 'mediaAttribute' => $mediaAttributeCode);
}
}
}
This code seems to simply set a single file that later gets added to the product data. What you could do would be to rewrite this class and update the code to deal with more than one product image.
Something like the following may point you in the right direction.
$allImages = explode(',', $importData['media_gallery']);
foreach ($allImages as $file) {
$imagesToAdd[] = array('file' => trim($file));
}
$mediaGalleryBackendModel->addImagesWithDifferentMediaAttributes(
$product,
$imagesToAdd, Mage::getBaseDir('media') . DS . 'import',
false,
false
);