sphygmos

I figured it out on my own. First off, if you get “Filetype not supported,” you need GD support for php. If you installed php from rpm, you need to install the php-gd rpm as well. If you installed from source, you need to recompile with GD support.
As far as the blank page after installing gd, I checked the httpd error log and saw this:
PHP Fatal error: Allowed memory size of 8388608 bytes exhausted (tried to allocate 12000 bytes)
I had to edit my /etc/php.ini file and increase the memory_limit variable to 24M. Now it works!