So it's not as important, but effectively you would find a small linux suitable for an old laptop, such as Damn Small Linux or one of its compatriots. You would install it on the flash card, and then get a graphics/slideshow program -- ideally one that runs on the console frame buffer, not on an X server as that takes memory and complexity. Then set up that slideshow to run from a slide directory, and fill that with slides. Then set things up in your /etc/rc.local or other boot script to run the show on boot.

Next, I would put in a script that runs frequently from the cron to rsync the slideshow directory from a master source if you have networking on the laptop, and then you can update the master directory and the slideshow will change.

Or write a script to suck down an RSS feed with pictures, such as a flickr feed, and have it run from the cron every so often.