I had all the same issues. I tracked down a solution. The NVIDIA logos are BMP3 images. I was making BMP images. I was taking a png, and converting it with imagemagick.
convert -resize 1920X1080! untitled.png ./nvidia-uefi/edk2-nvidia/Silicon/NVIDIA/Assets/nvidiagray1080.bmp
I do this and it works great!
convert -resize 1920X1080! untitled.png BMP3:./nvidia-uefi/edk2-nvidia/Silicon/NVIDIA/Assets/nvidiagray1080.bmp