Well, seems that at some point between PHP 4.4 and PHP 5, something about my libgd, libpng or zlib broke, because now I'm getting all these
PHP Fatal error: imagepng() [<a href='function.imagepng'>function.imagepng</a>]: gd-png: fatal libpng error: zlib error in ...
errors popping up. I know it worked a while ago. Not sure why it's not working now though.
Great, something else to dig into. One of these days I'm just going to have to break down and redo the whole server.