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I don't think you will ever get this machine to boot off a thumb drive. Unless it's supported by a machine's BIOS, there's pretty much no way to do it.

Although it is definitely possible to achieve what you're looking to do. You'll need to get some sort of OS on there- maybe an archaic version of Linux? As well as the X Window system and a decent window manager. I just hope the screen's resolution is good enough to show a downsized version of the pictures...

Without a modern linux kernel, I don't think you're going to get it to read a thumbdrive through a PCMCIA usb adapter (if you can even find one, and if this machine has said slots).

Keep in mind the pros and cons of spending time on it- I mean, if it's worth your while as a learning experience, definitely go for it. It'll be a challenge, and I'm pretty sure this is possible. But anything you'd need to buy to make it happen would probably be around the cost of a cheaper digital frame.