FBInk also ships with an utility that will allow you to painlessly deal with the insanity that is framebuffer layouts on Kobo.
e.g., at a glance at what's happening here, something like `fbdepth -R CW`.
On the upside, fbink itself will also allow you to display an actual sane image format (i.e., PNG) without having to do the insane and messy conversion to the matching raw pixel format that's happening in that link.
(Which, btw, is not "proprietary", it's RGB565 ;p [for the device in question; all Kobos still boot @ 16bpp, but it's been quite a few years now since Nickel itself moved to RGB32 @ 32bpp. Since pickel is designed for that early recovery & boot progress bar stage, it still targets that legacy layout, though. TL;DR: Don't, just use fbink
)