How do I backup photos *from* iCloud Photo Library
- If the photos you want are on iCloud:

Download the files as follows:  


Launch the Photos app
Go to the Photos > Preferences 
Click on the iCloud tab
Click on Download Originals to this Mac


If you hard drive is full, then before doing this change the location of the iPhoto Library to an external drive.  Instructions for that are here: http://appleinsider.com/articles/15/01/25/how-to-safely-move-your-macs-iphoto-library-onto-an-external-drive

See below to get specific photos in the iPhoto library. 

- If the photos you want are currently on your hard drive:

Copy or set your backup software to copy the following item:

/Users/your-username-here/Pictures/iPhoto Library

That should do it.  

This is not a standard file, it is a package, so if you want to copy specific photos you can control click the file then choose "Show Package Contents".  You will then see a folder with various items including subfolders that contain your photos.  I believe the originals are stored in a folder called Master and are organized by date.  You can copy what you want from there.  Some back-up apps will even allow you to sync these subitems as opposed to the entire package.