ofn-brush-strokes-on-path

[denzjos]

Ofnuts, 'ofn-brush-strokes-on-path', a nice script. Two problems : if I animate a scene in gimp, then the moving object is perfect blending out. When I export the animation to a gif file, then the blending out disappeard. What am I doing wrong in the workflow ? Second problem : how can I use a background (picture or photo) in the animation? I just can choose between white, background(colour), foreground(colour), pattern and transparent. I tried some things but I can't figure out how to do it. Thanks in advance for the answer.
The exported gif file :
   
As seen in the animation in gimp:
   
Gif export settings I used :