[neil_swann80]

You could also try this workaround by editing the coverimage.koplugin:

Exit from Koreader.
Add entry to: /mnt/ext1/applications/koreader/plugins/coverimage.koplugin/main.lua
The entry you need to add for the power-off logo is highlighted in red below, the green is for the boot logo. You can find the point to add them at around line 119.

function CoverImage:createCoverImage(doc_settings)
    if self:coverEnabled() and doc_settings:nilOrFalse("exclude_cover_image") then
        local cover_image, custom_cover = FileManagerBookInfo:getCoverImage(self.ui.document)
        if cover_image then
            local cache_file = self:getCacheFile(custom_cover)
            if lfs.attributes(cache_file, "mode") == "file" then
                logger.dbg("CoverImage: cache file already exists")
                ffiutil.copyFile(cache_file, self.cover_image_path)
                ffiutil.copyFile(cache_file, "/mnt/ext1/system/logo/offlogo/koreader_cover.bmp")
                ffiutil.copyFile(cache_file, "/mnt/ext1/system/logo/bootlogo/koreader_cover.bmp")
                io.popen('iv2sh WriteStartupLogo /mnt/ext1/system/logo/bootlogo/koreader_cover.bmp')
                lfs.touch(cache_file) -- update date
                return
            end

Try changing books in Koreader to generate the new cover.
Exit from Koreader.
Then try manually changing your power-off logo to: koreader-cover
Power off the device to check the logo.

* This may stop working after a Koreader update as the plugin may be overwritten; you'll simply have to re-add the lines above.