If a content type, like the Photo Gallery page, offers you the ability to upload an image–specifically outside of the WYSIWYG add image option–you will have the option to upload more than one image at a time, making the process more efficient.
The slideshow gallery block is a system block (View) that will pull in information from an already-created Gallery page in your site. If you don't already have a gallery, learn how to create a gallery page now.