To set a custom boot logo, place a photo on the USB drive, select it in the media player, and use the Capture Logo option in the service menu.
This community forum provides a post titled "T.RD8503.A8 USB Firmware Software Bios Bin" which explicitly lists the 1920x1080_USB version.
Extract the .bin firmware file (often named SKR8503_100.bin or similar) from the archive.
Wait until the flashing stops or slows down (usually about 1 minute). Power off the board and remove the USB drive.