I did a search of the filesystem using Better Terminal Emulator Magic. There are a lot of directories I didn't have permission to view, so I likely missed something. Couldn't find any jpeg or gif files other than what I have on the external SD card. It did find a lot of png files. Haven't have time to inventory them, but they mostly appear to be icons for the reader and journal.

Update: Did not find any png files on the system that were anything other than button/icon graphics for the reader and journal.

I found some links about changing the boot image:
http://forum.xda-developers.com/showthread.php?t=443431
http://android-dls.com/wiki/index.ph...ck_Boot_Images

The picture is not a simple png/jpg/gif. And it has to be extracted from the boot image file, modified, and reapplied to the boot file. However, the picture discussed in these links may not even be what we're looking for. Since these links discuss the "standard" Android implementation this information may only apply to the picture that appears on the LCD side at boot time.