I got my splashscreen back. I don’t know why it’s harder now but it looks like they moved all splashscreens to /opt/retropie/supplementary/splashscreen/ which you don’t have permission to mess with unless you create a root password and login with root.

So upload your custom image somewhere in retropie where you have permission and then go to the setup script, choose custom splash, option 5 manually set splashscreen list then paste in the path to the image, like
/home/pi/RetroPie-Setup/supplementary/mczelda/mczelda.jpg

This seems much harder to me than having them all just sit in the supplementary dir like they were in BETA 4 and before…