My Photos.app starts on system startup
You probably have a SD card or a device which stores photos plugged in your Mac.

To prevent Photos from auto-opening, follow these instructions:


  
  Connect the iPhone, camera, SD card, etc to the Mac and let Photos app launch itself as usual
  Under the “Import” tab of Photos app, look in the upper left corner to find the device name, this will indicate which hardware will no
  longer automatically activate Photos app
  Click the checkbox so that “Open Photos for this device” is no longer selected (likewise, toggling this on would enable Photos app to
  open itself again automatically when this device is connected)
  Quit out of Photos app, the change is immediate for that device – this can be switched back at any time in the same OS X Photos app
  
  
  


Via: http://osxdaily.com/2015/05/31/stop-photos-opening-automatically-mac-os-x/