I found a quick-and-dirty workaround:
in the file nggfunctions.php, I found the lines:
// generate the thumbnail size if the meta data available
if (is_array ($size = $picturelist[$key]->meta_data[‘thumbnail’]) )
$thumbsize = ‘width=”‘ . $size[‘width’] . ‘” height=”‘ . $size[‘height’] . ‘”‘;
I commented out the last two, so that the thumbnail size is not taken from the meta date but from the preset thumbnail size. Not elegant, but it works. Anyway, a fix for proper thumbnail size would be better ??