On newer versions of Windows DirectX is not available when no user is logged in for this reason only Standard 2D transitions works on logon screen saver. In addition most video players use DirectX for this reason generally it is not possible to play video from a logon screen saver.
VLC Player is able to play video also without DirectX, you could try to install it and configure gPhotoShow to play all video with VLC
gPhotoShow displays a black screen when it can't load any file. When started as logon screen saver gPhotoShow runs under a very limited user account so it is possible this user hasn't the rights to read the configured folders. Try to add some pictures in a folder created directly under the root of the C drive (c:\mypics), add this folder to gPhotoShow and set it again as logon screen saver. If these pictures are displayed it means the other folders you have configured can't be accessed by that limited user, you have to check permissions of those folders and make sure "everyone" has read permission.
