Did not care much about the boot image feature of the pocketbook until I actually saw them. May the fellows forgive me, but the default images are... not very pretty.
And the cat image from
http://www.greenreader.info/home/resources-en is.
But it's in the wrong format. You need BMP. In 16 shades. Or 4 bits.
To achieve that - on Linux - use Gimp's
Colors->Desaturate to make it black and white
Colors->Posterize, 4 levels and save as Windows Bitmap