So I asked Dax at discord on reswitched how to change it and he gave me these instructions.
the sept splash can't be changed or removed, but if you mean the second Atmosphere splash, and you're using fusee to boot, then you can edit /atmosphere/BCT.ini; add a section like this (you can change the path and filename to whatever, it just has to be a bmp file)

[stage2]
custom_splash = /path/to/your/bootlogo.bmp

As for the bmp file itself, it needs to be 720x1280 (so 1280x720 rotated 90 degrees counterclockwise/anti-clockwise) and exported as 32-bit ARGB

I will try and reply!

 

[pohaxsf]

WELP, I am getting color depth error, so I will investigate ams.

 

[pohaxsf]

welp
my photoshop has disabled alpha channels
but if I open the atlas one and copy it
it WORKS

 

[ScarletDreamz]

This has been already added on several boot screen threads.

 

[pohaxsf]

This has been already added on several boot screen threads.

provide link, it's N o w h e r e on guides

 

[ScarletDreamz]

provide link, it's N o w h e r e on guides

https://gbatemp.net/threads/share-your-custom-atmosphere-fusee-secondary-bootlogos.532018/

 

[pohaxsf]

https://gbatemp.net/threads/share-your-custom-atmosphere-fusee-secondary-bootlogos.532018/

Nvm

 

[ice_monk]

Thanks for this.