You could use a class to mark the images to exclude in your lightbox, and those images will not be used in the lightbox.
To mark your images and then initialize the lightbox with this selector
<script type="text/javascript">
$(function() {
// Mark unwanted images
jQuery('#gallery a[href*="noimage.png"]').addClass("no-lightbox");
// Initialize lightbox without them
jQuery('#gallery a:not(.no-lightbox)').lightBox({
overlayOpacity: 0.6,
imageLoading: 'LightBox/images/lightbox-ico-loading.gif',
imageBtnClose: 'LightBox/images/lightbox-btn-close.gif',
imageBtnPrev: 'LightBox/images/lightbox-btn-prev.gif',
imageBtnNext: 'LightBox/images/lightbox-btn-next.gif',
fixedNavigation: true,
txtImage: 'Image',
txtOf: 'of'
});
});
</script>