I am noticing that some of my images are not displaying (shows the icon for a broken link). I've verified that the image exists, and I even followed the steps to change the php.ini file. I still get the issue. I've re-sized some if the images as a test, and this does work. My question is what is the limitation for image generation in zenphoto? Is it based on total pixels, or width/height? I would like to know before I go through and batch re-size my photos. Or if there's something I can change in my theme (zp-skeleton) or the zenphoto installation to allow for all image sizes. Thanks!

The limit is your server, not Zenphoto: http://www.zenphoto.org/news/problems-with-albums-and-images

Is there a way to find out what my server limits are?

It should be listed on the admin overview page. Otherwise your host should know.