It’s quite simple. Download the repository. And prepare everything to compile it. We have a tutorial for it here:
https://www.bforartists.de/data/tuts/Building_Bforartists_with_Cmake_and_VS_2017_on_Windows.pdf
But instead of vs 2017 you should use vs 2019. The tutorial needs an update here.
Have a look in the release\datafiles folder. Find and open the splash_template.xcf. That’s a gimp file.
Change the image to your needs. Export the result to splash_2x.png and splash.png in the given size.
Now you simply compile Bforartists. And the new image should be in place.
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