Just tried, works here on FW U700.6.8.4183.
Since we have so few "ready-made" images, I’m adding the one I currently use on my Pocketbook Era, a sleeping cat.
@Tintenfisch: Would you share your sleeping cat? I quite like that, too.
Re your question: Didn’t try 24-bit, I saved mine as indexed 16-color, this reduces filesize from 6.4 MB to 1.1 MB.
Code:
$ file taskmgr_lock_background.bmp
taskmgr_lock_background.bmp: PC bitmap, Windows 98/2000 and newer format, 1264 x 1680 x 4, cbSize 1061962, bits offset 202