When starting up DVD slideshow GUI will convert all bitmaps/avisynth scripts in the Transitions folder to transitions. So by adding a bitmap/script to this folder and restarting DVD slideshow GUI a new transition will be added to the transition options.
Transitions can be made out of gradient greyscale images going from black to white. You can use the freeware paint programs like Paint.Net or Gimp to make gradients.
Transitions can also be made out of Avisynth scripts. An avisynth script is a txt file with an .avs extension. Open one of the avs files in the Transitions folder to see an example.
The variables which must be used in each script is at, bt and ft.
Ex. This is the code from the Wipedown.avs file:
# TransAll avisynth plugin created by V.C.Mohan
See the transall.txt file to look at more examples of different transitions.