Dunno about a slide show program, but remember this is a 68040 processor so you'll have to avoid any software that requires a PPC.
As far as the battery charger goes, good luck finding one. VST made the most common charger, but stopped making it a long time ago. You can find them on eBay for about $75-$100. I'm looking for one myself but there is no way I'll pay those ridiculous prices. At the same time batteries are ~$50 a piece so if a charger keeps them from going bad, it may be worth it.
There is also some shareware software that will drain batteries and hopefully breathe life into them: http://members.aol.com/jbkezer/amnesia.html
But first use VST's Emmpathy software to check the batteries. I can't find it right now but dig around and you'll find it. If you're on eBay, pick up an AAUI transceiver so that you can connect your iMac and PB520 - then you can use the Powerbook as a cheap floppy drive!