Resolved: Change Initial Splash Screen background
Marcel told me in private and I don't miss the opportunity to make it in
public. Notice that the image is 640x400.
I found a way to do that.
Create your image with gimp.
Convert to 14 indexed color (Image > Mode > Indexed).
Save as ppm
ppmtolss16 '#d0d0d0=7' < splash.ppm > splash.rle
I had to instal syslinux package to have the ppmtolss16 command.
Copy the new .rle to the isolinux template folder and rebuild.