How to download a webpage's all images at once?
Automator
Use OS X's Automator.app to find, extract, and save the images from your current web page. The combination of Actions needed are:

Get Current Webpage from Safari
Get Contents of Webpages
Save Images From Web Content

To learn more about using Automator, see Apple's Mac Basics: Automator.

Terminal
An alternative approach is to use curl through the command line, What's the fastest and easiest way to download all the images from a website.