For a lot of people, pictures are the things that eat up space on their internal hard drives. Add in the more frequent use of solid state drives with relatively small capacities on the new MacBooks and this leaves people needing external or cloud storage for their photos. I personally recommend exporting your entire iPhoto or Photos library to a external hard drive. The major upside is you have more internal storage for apps, documents, and other media. The downside is you need the external hard drive to access the library.
To do this the first thing you need to do is purchase the drive you want to use. If it is not already formatted for Mac, you will need to reformat the drive as Mac OS Extended (Journaled) in Disk Utility. Then you select the current iPhoto/Photo library and drag it to the external drive. When you next open up the Photos app you hold down the option key. You will be able to choose the newly created library for the app to refer to by navigating to the external drive. I highly suggest you use the application and check to make sure all your files are there. After testing the library, and backing up via Time Machine (or other methods) to a different drive, you are free to trash the original and free up internal hard drive space.