Every store and all its elements, be it products, logos, collections, homepage, promotions, banners, etc. needs to be presented well and images plays important role in it. And uploading each image one by one is time-consuming (if only you have one element, that's another story). therefore, Gallery allows you to upload all kinds of images at once and categorize them in their respective section (i.e. Products, banners, or other images).
Using gallery, after uploading pictures you can then associate them to your products as you like. To know how to upload images follow the steps:
Go to Gallery, then select from products, banners, logos, or other images tab so that you can divide them right away.
Click on the Upload File button and locate your image folder and then simply click on open/upload.
You can simply drag/drop your images in the gallery section too if you want.
1. The maximum size of an image is 10MB.
2. The dimension (or aspect ratio) varies according to image type (product, banner, logo, etc.) you can read more about this, here Design Handbook3. while using the images for bulk upload of products through CSV, use the name same as it was of the image you uploaded in the gallery, to avoid any errors.