If you are using Mac and you get your photos sync via iCloud, you are usually open/view the photos in the Photos app. If you need to modify a photo using the Preview tool you have to:
1. Export your photo from Photo App to your local drive (File -> Export -> Export 1 Photo…)
2. Select your photo format (kind) and click Export
3. Select your location on the drive and click export
Now you have a copy of your photo on your local drive … open it with Preview or any other photo viewer/editor.