Oh, I never really thought about the headers of the XML document, but I'll include that later today, and probably make a minor updated version which the few corrections so far.

Btw, is there anywhere where the headers are important?

header('Content-Type: text/xml;charset=utf-8');

just after:

/**
* At this point we are going to try to create the sitemap!
*/

The '?sitemap=root' page only holds information about the front page of the gallery. This is since I'm using a sitemap index as the root sitemap, and that doesn't allow me to put in page entries on the root sitemap.

In order to get the front page indexed, I then have to make a separate sub-sitemap for the front page, since it otherwise should be included in one of the sub-sitemaps for an album.

I have Google using the sitemap as well, and that doesn't complain about the '?sitemap=root' sitemap, however, I can see what it's referring to. There may not be a reason for supplying a priority to the single page in the sitemap, since it can't give a relative priority, if there aren't any other pages it can compare the priority to.

A possible easy fix would be to comment out/delete line 64 in the plugin (echo "<priority>1.0</priority>\n";) and see if that helps out.

I'm using this plugin now and added the sitemap to my google account.
It seems like everything is fine, except for ?sitemap=root. Here Google gives me a warning which I will try to translate from dutch into english ;) :

'All URL's in your sitemap have the same priority (not the standard priority). Priority will tell Google about the importance of one URL over another. If all URL's have the same priority, Google doesn't know which one is more important'.

I don't know if this can do 'harm' to how the sitemap functions or that I could be banned. I guess not, but perhaps this ?root isn't even necessary?