OK. So, I've fixed the problem for myself. I'm not a PHP programmer at all. But, after some googling, I decided to encapsualte all of the calls to load "/lib/image.php" with an if statement...
if (!class_exists("nggimage"))
This fixed the first error, but revealed another, so I did the same for "/lib/meta.php"...
if (!class_exists("nggmeta"))
I'm sorry, but it would take me too long to explain which files, and which directories to put them in, etc. that it requires to make it work. So, I just posted the files and directories that need to be replaced. You can download them here if you want...
http://www.jeffreyconway.com/nextgen_gallery_fixes.zip
I know it's cludgey. And, I don't guarantee any of the work (like I said, I don't actually know PHP) but it's working for me. I hope this helps someone.
-Jeff