Aura HD Customizing the sleep screen

Hi list,

I am trying to customize my Kobo Aura Hd sleep screen. I don't know how it's called, but I mean the image which shows when I put the e-Reader to sleep.
In my case, it's a logo of FNAC (French shop selling the Kobo).

This URL explains how to replace that logo with the Kobo logo. But that's not exactly what I want: I want to replace with an image of my own. Right?

Also, with help of people of this forum, I managed to customize the demo slideshow of the reader, so that it displays images of my choice. But that's not the sleep screen...

Anyone help me out please? I have ssh access to my e-Reader and thus can hack/modify files.

Cheers,
Axelle