Because the Flash Lite implementation (version 1.1) on the iRiver U10 does not support loading raw JPG files, all pictures that you transfer to the U10 are first converted to Flash SWFs and renamed with a JFU extension by the iRiver Picture Plus software. By overwriting these files with custom Flash SWF files, it is possible to have much richer background content for the U10 than just static images.
I've created a few sample backgrounds that have an analog clock with date and rotating images in the background. Because of the performance limitation of the device, backgrounds with too much animation or processing will significantly slow down the interface navigation. I've found that running the analog clock and rotating the background image every 20 or 30 seconds is about the limit of these customized backgrounds before the device becomes difficult to use.
Free custom backgrounds for U10 (install instructions in ZIP):
Yosemite Slideshow Background

Download yosemiteSlides.zip (135KB)
Neon Genesis Slideshow Background
Download neonGenesisSlides.zip (76KB)
Enjoy!
(Thanks to Justin Everett-Church for finding this hack! :))