I think it is an error in WordPress 2.1 because I have the same problem now with Subscribe 2.3.0 for WP2.1. Subscribe also adds a new icon to the editor...
A new version of WP has been released this morning: 2.1.1 and the editor is mentioned as one of the bugfixes in this new release. I'll give it a try later on. But now I have to do some REAL work (haha).
OK, I never used a photo plugin but I downloaded Zenpress 1.2.2 from the Wordpress site and installed it. It was one folder you FTP into the WP plugins folder.
Activated it and filled in the url and path. It worked fine. I'm running the latest WP 212.
One thing I didn't see was a link back to WordPress from Zenphoto. Maybe I'm missing something.
zenpress and lightbox together would not be necessary, as zenpress is specifically for inserting zenphoto images into wordpress posts. You would be better off with lightbox after the images are inserted into wordpress, and there are plenty of lightbox-ish variants for wordpress out there.