Apple - How to Install pdfimages in El Capitan?
You can install via Homebrew with the following command:
brew install poppler
Another command-line program that can extract bitmap images from PDF files is the command-line version of the Unarchiver app called 'unar'. (Doesn't need Homebrew or anything to install.) The command is simply
(The original GUI-based Unarchiver.app does this as well.)