Trying to answer your questions one by one. I do not know about that particular machine and it's BIOS, so a lot of it is guesswork; but it might help you to get closer to a solution.
- Why is Plymouth displaying the wrong image?
Maybe there is more than one entry in the UEFI BIOS' ACPI "BGRT" (Boot Graphics Resource Table) and Plymouth (probably more likely, the kernel, see below) is selecting the wrong one.
- How does Ubuntu pick the logo image from EFI?
The Kernel provides a view onto the BGRT in
/sys/firmware/acpi/bgrt/image being the BMP image to display. You can copy it to e.g.
/tmp/bgrt.bmp and open it from there to see what it looks like.
- Can I help it choose the correct logo image to display during the boot process?
- Or even better: can I customize it?
Probably the easiest way is to provide your own image and load it instead of the BGRT image.
The quickest way, IMHO, is to comment out all the
UseFirmwareBackground=true lines in
/usr/share/plymouth/themes/bgrt/bgrt.plymouth and place a screen-sized (e.g. 1920x1080) PNG with the logo you like as
As your modifications to
bgrt.plymouth will likely be undone by a later update, you might want to copy it to
/usr/share/plymouth/themes/fixed-background/fixed-background.plymouth and switch to it with
update-alternatives --config default.plymouth, and make it available for future boots with