Well, earlier.
I have remastered core.gz for this. I've added /usr/local/bin/fbv and of course edited/added scripts to use it.
However, I'm not sure if you would want to edit tc-config and remaster core.gz like I did, as you want a digital picture frame, not a boot animation.
I guess a slide show could be shown as early as the first tc-config messages are displayed though. But I'm not sure if it would be worth the hassle instead of just waiting a few seconds for the boot to finish and the slide show to start.
If you really want to do this I could help (if I don't forget about it again :p).
In the mean time, this would work for me
/usr/local/bin/fbv -a -i -e -s 100 /opt/backgrounds/*.* 2>/dev/null
Probably at the end of ~/.profile .