Using photo editing apps on OS X El Capitan 10.11.6

Hey! I use an older mac (MacBook Pro mid 2007) running 10.11.6 and I've been trying to get photo editing apps on it but I can't seem to find any that work on this older OS, neither from the Mac App Store nor the internet. Does anyone know of a (free) software that still supports 10.11.6? Or any old version that is available to download for that matter? Thank you!