From the two last SEO Yoast plugin upgrades, I got errors in XML sitemap of that plugin. I think the question is that <image:loc></image:loc> tag in sitemap is empty, instead of having a link to images.
Issue appears only to Nextgen Gallery images, not in default wordpress ones, and I'm not the one with this issue.
Of course first I contacted SEO plugin's author, and after a week of attemps he address me to this support, with these words:
"He hooks into my API but apparently does so in a wrong way sometimes. Going to resolve this as it works fine with normal galleries"
I have 1.055 errors in sitemaps with Netgen images, detected by Google Webmaster, as follows:
"Invalid URL. This is not a valid URL. Please correct it and resubmit. Main tag: image, tag: loc"
446 errors are in post-sitemap and 609 are in page-sitemap (where I have galleries)
Here some screen shots:
I hope there will be a fix for this.