This can help. (this will help)
http://wordpress.org/support/topic/nextgen-gallery-yoast-xml-sitemap-broken-links?replies=2
Images will be included in page-sitemap.xml – forget about ngg sitemap it will create broken links.
I am assuming you’re using Yoast SEO
Hmm, I just tried WordPress(aka Yoast) SEO with ngg_tag taxonomies turned off and it certainly did not produce an image sitemap of NextGEN gallery images.
Unfortunately, to the best of my knowledge nothing picks up the jpegs etc in NextGEN galleries and adds them to a sitemap.
I’d VERY VERY VERY happy to be corrected on this. I should note my gallery setup is a little different to most.
