Well, afaik, if you enter the test urls in and they come up blank, it means that the server you're attempting either isn't installed or it isn't working right.
Which would mean that either the path to your imagemagick binaries isn't properly set, that it isn't installled, or something is broken. For GD it means that PHP wasn't compiled with GD Support (if it was installed via yum then you may need to download the appropriate module).
