WelcomeSplash-dfsg.ppm is rendered from version17.svg/xcf, which does only
contain freely usable boring artwork made by ourselves, iow. completely
freely redistributable for any purpose.

version19 is the most recent version of the WelcomeSplash-upstream.ppm
Designed in Inkscape by Celia Foster, saved as .svg
Opened in GIMP, saved as .ppm
The frog image is from pixabay, licence is free for commercial use,
no attribution necessary (see screenshots for details). 
https://pixabay.com/vectors/frog-green-animal-amphibian-48234/ 
We are told this is not Debian dfsg compliant.

version17 is the most recent version of the WelcomeSplash-dfsg.ppm
Designed in Inkscape by Celia Foster, saved as .svg
Opened in GIMP, saved as .ppm

Size of a splash must not be bigger than 1280x1024 as of PTB 3.0.17 from early November 2020.
The actual copy to be used as splash screen must be copied into Psychtoolbox/PsychBasic/
as WelcomeSplash.ppm
