If you've Time Machine backup from prior to upgrade El Capitan, that should have iPhoto.
The last iPhoto 9.6.1 version was still available after upgrade to El Capitan; and can be used
in this Mac OS X alongside the system's newer Photos, there. They have different libraries.
Some research would be involved to see where the last iPhoto may be found at this late date.
https://duckduckgo.com/?t=ffab&q=how+to+restore+iPhoto+in+El+Capitan+after+upgrade&ia=web
Mine was easier to accommodate because original Mavericks and El Capitan ~ were both on
one hard drive, in separate partitions. (Second internal matching HDD has High Sierra 10.13.6
with 'extra partitioned space' to also consider Mojave 10.14.6.) Mine allows four builds macOS.
Other choices and access to internet recovery, macOS Utilities, and others, are available with
different key sequences on startup into Recovery. ~ This also works to add recovery to erased
wiped or new hard drive replacement.
• How to reinstall macOS from macOS Recovery - Apple Support
https://support.apple.com/en-us/HT204904
Given the lack of security updates for older Mac OS X; you may reconsider, check what's best.
Good luck & happy trails!🌞🐝