I don't know how to program, but I would have thought a simple program which had access to a directory with photos split into files of times (1,440 minutes in a day, ie: 1,440 files)

you could easily replace and get different calendars.. just cut and paste..

screen saver clock is the most obvious place for it for me

In car charger and desk stand with charger is probably the only way that the lack of battery life wouldn't effect me though..