When you bulk download a collection of images, they are downloaded as a ZIP folder to your local machine.
You can open a ZIP folder on a Mac using Stuffit Expander. Stuffit Expander is usually already installed on a Mac in Applications > Utilities. If you do not have the application already, however, you may download it for free from Stuffit.com.
To unzip the folder of images, simply open Stuffit Expander, and drop the ZIP file into the Expander window.
When you drop your ZIP folder into the expander window, the expander will generate a new folder with all the images ready for use.
Note that icons may appear as different images to the example shown above.
You might find that you need to change the access permissions to be able to open the files. You can do this by highlighting the folder of images, then hold down the apple key and hit the letter i. This will open the 'Info' window. From here you will be able to edit the 'Ownership and Permissions' area to give your self 'Read and Write' access.