I therefore opened
./usr/share/plymouth/themes/mint-logo
After I backed up the mint-logo, I edited it with my own logo.
I updated initramfs properly, and I shut my computer down, and my logo was showing during plymouth shutdown screen.
Now, when I started mint again, it would show the mint logo on startup, and mine on shutdown. Weird, eh ?
It gets more interesting : if during startup I press the right or left arrow twice, to enter and exit plymouth text mode, my logo replaces mint's official logo.
So, my line of enquiry is the following : what is wrong with my install ^^
(appart from the fact that it is both stupid, inelegant and wrong to change mint's theme instead of applying a new one)
Linux Mint 18, 64 Cinnamon, recently updated to 18.2, including migration to light-dm.
Has anyone got any idea how to solve my issue ?