The screensaver shown when you "turn off" the 650 (and I assume 350) is quite a nice extra feature. For those who haven't played around with this feature, I have some extra info.
1. The images chosen for the screensaver are selected in the pictures application. With this application open, you choose "options>select standby screen". This opens up thumbnails of all your images, and allows you to checkmark each image you want in the screensaver application. This also allows for more than one image to be chosen, which will cause the screensaver to cycle to a different image whenever the reader is turned off (not randomly).
2. Images are stored in "(root)>database>media>images". Just drag and drop. (The Sony Library software may do this for you, but I use calibre so don't have the Sony software installed)
3. Images are displayed at roughly 600x800, but if you look at the images Sony put in the directory, those images are over 1500 pixels wide, which means that the reader scales the images to fit the screen. I haven't tried really weird proportions to see how good the scaling is.
4. Kindle users have already generated many images that are equally useful for the Sony devices since the underlying screens are the same. One good site has over 800 screensavers to start you off and can be found here:
Personally, my favourite is attached below