Q: uninstall EI Captain

El Capitan has nothing to do with Classic.   The only way to uninstall El Capitan is to restore the backup you made prior to installing it.   For Mac OS X versions 10.9 and 10.10, Apple only has those directly available through special request of the Apple Store.    If your Mac came with 10.7 or 10.8 or 10.9, or 10.10, you'll have to request AppleCare to make it available first before you can upgrade to the next operating system.   If your Mac is older than July 20, 2011 and later releases of Macs that year, but younger than March 15, 2010, you'll need AppleCare to send you the original 10.6 installer disk to restore 10.6 if you didn't back it up first.  Otherwise you can buy 10.6 from the Apple Store online for a physical CD.

See this tip on where to post for the appropriate operating system.  Classic is 14 years old.

http://discussions.apple.com/docs/DOC-2463

Did you simply install El Capitan on top of Snow Leopard, Lion, Mountain Lion, Mavericks or Yosemite?

Are you using the last version of iPhoto?

I'm going to ask this thread be moved over to the iPhoto forum.