Lost photos when upgraded to mountain lion

So I had all of my photos stored in iPhoto, but my work upgraded my OS from Snow Leopard to Mountain Lion (yes, I realize Lion was skipped), I lost iPhoto along with all of my photos. iPhoto came pre-loaded on my machine, so I don't have the software to re-install iPhoto. Advice? I did save my photos on an external hard drive (via time machine) prior to upgrading, but I'm not sure if they'll be accessable since I don't have iphoto on my computer anymore. Thanks!