Here is the answer from the Libreboot chat.
> A: you need to get familiar with CBFS. "yourrom" means "your rom" = what
you have installed
installed = flashed
> ME: *kindly askes for further details / explanations*
> B: just leave it be if you're not willing to do your own research, "A"
pointed you in the right direction.
> A: "yourrom" means "your rom" = what you have installed
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Then there were some further comments that fall under "trolling" in my book,
but maybe I am wrong and the answer is hidden somewhere between the line:
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> A: you could also say that by installing something, you "put it in" to the
box you call a computer. If you put something in, and the box did not belong
to a quantum physicist, you can usually take it out again.
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Seems, the Libreboot community doesn't really help here. So I repeat my
question I posted in that (kind of hostile) chat:
I'd like to change my boot background image. I found a thread in the
trisquell forum saying "cbfstool yourrom.rom ..." does the trick. Alas, I
don' know where to find yourrom.rom. Any pros around who can explain this to
ppl that aren't familiar with the whole Libreboot matter?