Alright. Vanity got the best of me; I caved and bought this abandonware to find out how it ticks so I could make my bootloader pretty.
Logo Me dumps sbl (part of the bootloader), and only uses known bootloaders because the offset changed with each successive bootloader.
I've just figured it all out and I'm not sure why @Ezekeel didn't come up with a more elegant solution, since just like with the awesome imgdata work by @bitdomo, sbl images are all in a particular order, and, searching for them we can learn their offsets, and knowing their sizes we can actually change them quite easily.
Here's what I've got so far:
dd if=/dev/block/platform/omap/omap_hsmmc.0/by-name/sbl of=/sdcard/sbl.img
dd if=/sdcard/sbl.img of=/dev/block/platform/omap/omap_hsmmc.0/by-name/sbl
JPEG header magic:
FF D8 FF E0 00 10 4A 46 49 46
Different offsets with each bootloader since bootloader=gpt+xloader+sbl:
grep -obUaP "\x00\x10\x4A\x46\x49\x46" sbl.img # unfortunately doesn't work on Android grep
-4 to get to beginning of JPEG magic
KK15 (my favorite due to the better color calibration)
LC03 (last before the 2 second wait was added)
#11 image in sbl
720 x 595 pixels max
12126 bytes max
#16 image in sbl
720 x 100 pixels max
2215 bytes max
00 padding to full byte length (if necessary)
FF to 4 trailing bytes
I used Photoshop to reduce the size of the latest Google logo and lock icon down to the dimensions of the GN versions, then TinyJPG.com to shrink them to (less than) the appropriate file size maximums. The FF trailing 4 bytes is something Logo Me does that it's unclear why, since there is usually other data there, but it works either way.
Also, important to note that flashing a bootloader.img will of course reset the changes, and that since bootloader.img files reset the GPT (partition table) any changes made by something like @Lanchon's brilliant RePIT will get reset as well. Logo Me especially doesn't cooperate with that, since it looks for a bootloader checksum which is of course affected by the changed GPT.
Anyway I'll probably turn this into a script soon enough, and provided the supplied jpg files are small enough it should be able to patch the sbl partition on-the-fly, and that should be friendly for repartitioned devices as well.
@bamtan2, @MWisBest, @Ziyan, @aosp, @bsmitty83 - I'll mention you since this thread is lost to the GN community way out here in the boonies of Paid Software.
As it is now, those osmod sbl dumps can be flashed from sdcard using dd to get the new Google/lock images into the corresponding bootloader (so flash that first if you want to switch), including the MD04 bootloader Logo Me doesn't support, and it works on repartitioned devices, so my goal is pretty much done.
So, for example, if you are on bootloader KK15, the root command for flashing my KK15 sbl dump osmod would be:
dd if=/sdcard/kk15-sbl-osmod.img of=/dev/block/platform/omap/omap_hsmmc.0/by-name/sbl;
I'll mention a few more.
@massoudsh @musical_chairs @zzpianoman @Fenix46 @rahimali @nikodr @rajputrajat @benkxda
Edit: Script now posted below!