And you're SURE you don't have iPhoto *at all*? That seems awfully unusual.
I'm guessing by your post that you also don't have the original restore discs for that machine (you could reinstall iPhoto from there). You may want to contact Apple, they will mail you a replacement set for a small fee.
Or as has been suggested you may just want to move to Snow Leopard, $30 if you can find a copy, and then buy iPhoto on its own for $15 (these are US/CA prices, YMMV).
While Picasa et al are not without their charms, iPhoto is to my way of thinking the best overall consumer photo-organizer and light-duty editor combo program on the market. There is SO MUCH you can do with it, particularly once you grok the idea of letting the computer handle the organization (this also applies to iTunes). For many people, no external photo editor is even necessary (though there are many REALLY good ones for the Mac).