It seems to be that this must be in gzdoom.pk3. It doesn't seem possible to put a banner.png into an add-on pk3 and have it work (though it will if you add it to gzdoom.pk3 directly - not a good idea unless you are making a custom game with its own exe and support pk3).
I'm assuming that this is because the iwad dialogue happens so early that nothing is else loaded and so it specifically looks in gzdoom.pk3.
Is that correct?
Not actually in gzdoom.pk3, but in vkdoom.pk3, in the graphics folder, there is a file called bootlogo.png. This is a graphic shown in the middle of the screen when the small red progress bar moves across the bottom of the screen when a Doom game loads.
I have tried putting a bootlogo.png file into an add on pk3 and it also works in gzdoom. Was leaving one out of gzdoom.pk3 intentional, or an oversight? The one in vkdoom.pk3 is a 256x256 graphic (other sizes work too). It strikes me as the GZDoom logo could easily be included as a bootlogo graphic (although the blue logo might clash with the red progress bar

I also noticed that, when games such as Heretic, Hexen and Strife startup, the bootlogo graphic is never seen. I assume that this is because those games have their own dedicated startups and there simply isn't the need or time to show an additional graphic?