Load your desired picture. Ensure the image resolution matches your intended boot resolution (commonly 640x480, 800x600, or 1024x768).
GFX-Boot Customizer is a Windows-based utility that simplifies the creation and modification of a "message" file. In the context of bootloaders like GRUB and Grub4Dos, the "message" file is a specially packaged archive containing all the assets for a graphical boot menu, including background images, font files, configuration settings, and panel layouts. gfx boot customizer 1.0.0.6 285
However, for daily drivers running Windows 10 or 11, or any machine with UEFI and Secure Boot, this tool is dangerous. It simply was not designed for that ecosystem. In those cases, look toward HackBGRT or leave the boot screen untouched. Load your desired picture