Transfer pictures via iCloud to new Mac
If the photos were in Photo Stream, then you can go to System Preferences → iCloud, log in and check the box for Photos. Then, open iPhoto/Aperture and let it download the photos.

If the photos were added to iCloud by a specific app, go to System Preferences → iCloud, log in and check the box for Documents &amp; Data, then open the app that you used to back up the photos.

If the photos were manually backed up to the Mobile Documents folder, go to System Preferences → iCloud, log in and check the box for Documents &amp; Data, then wait for your files to sync.