I have a NUC8i7BEH, which comes by default with some variation of:

Intel® published a way back the Intel® Integrator Toolkit (or ITT for short) which provides an UEFI executable that can be used to replace the default splash image. Let’s see how.

Requirements

NUC8 or older

The list of compatible devices can be found on the download page of the ITT.

A JPEG image with these restrictions:

Size

At most 60 KB.

Minimum resolution

120 x 120 pixels.

Maximum resolution

1920 x 1080 pixels.

JPEG Format

Unspecified. And only a subset of all the variants is supported. Fortunately, the ITT comes with a bunch of compatible JPEG files (the default one, at different resolutions), and we can use those as reference.

GIMP

We’ll use it to edit the desired picture and export it so that it adheres to the “specs” mentioned.

Producing the image

You can either draw one from scratch, or fetch one from the WWW via image search. Remember: nothing too fancy; we only have a maximum payload of 60 KB.

I’ve had success using images with black background, since it meshes nicely with the rest of the screen, rather than white, which produces a noticeable border when image ends and the black screen commences. But you be the judge.

Once you are happy, export it as JPEG, trying to reduce the size without sacrificing quality.

Producing a .bio file

1. Download the toolkit and extract the ITK6.efi executable onto your ESP. If that spooks you, you can use instead a FAT32 formatted USB drive.
2. Place a copy of the JPEG image alongside the ITK6.efi executable, e.g. splash.jpg
3. Enable the Internal UEFI Shell on the boot section of the BIOS Setup.
4. Boot the Internal UEFI Shell (pressing F10 on bootup may prove helpful).
5. Press ESC to prevent the execution of the startup.nsh.
6. Enter map -r to identify the device where the ITK6.efi resides, e.g. fs0:.
7. Enter fs0: (or the actual device in your case).
8. Enter ls to ascertain you are where you think you are.
9. Enter ITK6.efi -b -x SPLASH.bio -il splash.jpg. Some problems with the JPEG image are reported here, for instance, if it is too big. If so, rinse and repeat until this step does not croak.
10. Enter reset to reboot.
11. Press F7 to update BIOS.
12. Navigate to the ESP (or whatever device you are using) and select SPLASH.bio.
13. Confirm that you want to flash it.
14. The NUC will reboot, and you will be asked to confirm, by pressing 9, that you accept the flash request.
15. If the JPEG is accepted, you will see a message stating that the graphical memory is being updated. But if the image is not valid, it will skip to the (previous) splash image.
16. If everything is hunky-dory, the next thing you’ll see is your splash image, yay! But it could also be a blank screen. If so, try again until you get the JPEG format right.