Thanks for the quick response.
Ah getFullImageURL() sounds promising. I was hoping there was a built-in option I overlooked so I wouldn't have to go code hacking.
I think I want to make the modification in image.php and not album.php though. I don't want to break out of zenphoto and view an image directly; I want zenphoto to display the fullsize image instead of the larger-than-thumbnail but still resized image. Does that make sense? I don't think I'm killing any cool part of ZP.
I plan to upload images that are already the correct size for fullscreen viewing, so having zenphoto resize each image unnecessarily seems a waste of system resources and disk space in my case.