To create background contour pictures like this:
1) do loadpull
2) make contours with python script, and make sure they are square.
3) save contours as .png
4) open contour pictures in Gimp, and rightclick on the white background > colors > color to Alpha.  
5) rightclick on layer > scale layer > set width to 800 or 1000 pixels.
6) NB Here you can also correct the aspect ratio, by breaking the link between W and H,
but the picture will become blurry (unless you converted it to .bmp before scaling).
7) after the resizing, use image > fit canvas to layer
8) Then File > export as > xxxxx_alpha.png