Install ES File Explorer on your Bluestack.
To install ES File Manager on BlueStack Click On Magnifying Glass Type ES File Explorer. And Click On Install as shown below.
After Installation completes you will find ES File Explorer in My Apps. Click On it. As shown below.
When you click on "Whatsapp Images Folder". you will able to see all the images which is shared by your friends and family on whatsapp. Now select the images you want to save on your computer. If you want to select multi-images togather click on multi-select option as shown below. After you select images click on copy.
Now Go to your computer's C drive. You will find Folder with name "Program Data". It is below your "Program File" folder.
Click on Program Data.
Now Click on Bluestacks.
Click on User Data.
Click on Shared Folder.
You will able to find your images which you have paste in "bstshared" folder in ES File Explorer