Extract Images from Photo Libraries
Bulk extracting photos from a photo library is not normally needed since Photos and other library apps have a File > Export function. But there are situations where bypassing the library makes sense: a damaged or oversized library, migrating to a different app, or recovering images when the library database has been lost.
File Juicer can extract photos from photo libraries by scanning the contents directly, ignoring the database and internal structure. This means it works regardless of which app created the library. See also the iPhoto and Apple Photos page for more detail on Apple's library structure.
If you are only interested in image files, uncheck the other formats in File Juicer's preferences before extracting -- this speeds things up significantly.
Thumbnails and duplicates
Extracting everything from a photo library will produce duplicates. Photo library apps generate low-resolution previews and thumbnails for quick browsing, and File Juicer will find those alongside the originals.
Previews and thumbnails are much smaller files than the originals. Sort images by file size in Finder to concentrate on the full-resolution versions.
Forensics
Because thumbnails and previews are generated from originals, they can sometimes survive after the original has been deleted from the library. Photo library apps clean these out at unpredictable intervals so the reliability is low, but for forensic searches it is worth checking.