Why "not very lightweight and dependable"? If you take almost *any* popular GNU/Linux distribution and strip the bootscripts and kernel down to just what you need to launch your slide show (you could use directfb to go straight into the image display without loading a GUI environment), you would have a dependable slide show viewer that would boot up into the images in maybe ten seconds or so (depending on your hardware of course). But if you used a clean lightweight distribution like Slackware (or perhaps a complex distro that's been tweaked for fast booting, like Ubuntu) you might even get it down under five seconds: barely any slower than a dedicated photo frame. Plus, by using a proper full-featured Linux distro, you have so many other options: you could use a remote control to go backwards and forwards through the images, or even have the laptop respond to voice commands through its microphone (e.g. the spoken word "holidays" could switch to the holiday photos folders, "family" to the family folders, etc.). Admittedly the dedicated slide-show-distro you're imagining would take up a lot less hard disk space (because even a bare minimum install of a general purpose distro would include lots of tools you'd never need); but we're only talking about the difference between maybe 500MB for your dedicated slide-show-distro and 1GB for a stripped-down Slackware or whatever, and even very old laptops tend to have 40GB+ hard disks: do you really need the photo frame to cycle through more than 39GB of photos?
I suggest you choose a light distro like Slackware and strip it right down to the bare minimum, disabling all the boot services you don't need (even commenting out unnecessary sections of the boot scripts, if you like: Slackware's boot scripts are *very* easy to edit) and either building your own bare-bones kernel, or (if you're not comfortable doing that) at least blacklisting all the kernel modules you won't be needing (the laptop will have a bunch of things like networking hardware that you just won't need to use, so stop the kernel from loading the drivers for all that). You could also uninstall any software packages you know you won't be needing (though this isn't really necessary, so it's probably just running an unnecessary risk of crippling something, as I imagine you'll have plenty of hard disk space). Once you've got a system that boots very quickly, install a DirectFB-based image viewer (e.g. "DFBSee"...I've never heard of it before, but Google says it would work) and edit your .bashrc to mount any flash drive and display the photos on it, or (if there's no flash drive) display the default photos on the hard disk.
That's the basics and should only take an afternoon or so. The icing on the cake would be a pretty bootsplash (for e.g. Slackware or Gentoo) or Plymouth (for most other distros) animation showing a graphic of the photos loading (rather than the four or five seconds of text messages); plus whatever mechanism you want to put in place for pausing or selecting photos and so on. If you spent a couple of days on it, you could have a really nice solution set up, and all with an existing general-purpose distro. Then once you'd set it up once, you could just copy the whole thing onto any future laptops (making any necessary tweaks for different hardware).
Actually, even using directfb is probably caring too much about hard disk space. Distros like Ubuntu get to a full GUI so quickly these days, you might as well just use an x11-based image viewer if it has more features that you want.