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If the device generates the cover for an epub, it depends on how the cover image is displayed. Some people like to fill the screen even if it distorts the cover image. With this, the generated cover will fill the screen during sleep or the space it is displayed in on the home screen. Personally, I display the cover image so that it is not distorted. This generally means space down either side. And that is displayed whenever the cover image is needed.
There are two options when sending the covers from calibre. One will resize them to be 4:3. That will fill the areas the covers are displayed in in, but will distort the cover if it is not originally 4:3. The other option will send it and respect the aspect ratio. That means the covers are not distorted, but when used as the screensaver, doesn't fill the screen.
Kepubs are different. An image file inside the book is marked as being the cover. This is extracted from the file and used. It is used at whatever aspect ration so when displayed as a screensaver will not fill the screen.
The post GeoffR pointed to shows examples of the three different cases using the old home screen.
As to access to the hidden partitions, it depends on what you want. The firmware update package contains the contents of the rootfs partition. Just unpack it to see what is on the partition. Or you can enable telnet and get access to it that way. I don't have a link for this, but a search here should find it.
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