What file format are the bundles provided in?
Each bundle of images consist of a folder with a set of PNG files inside it. The bundle folder is compressed into a ZIP folder, ready for you to download. This is because compressed files can be downloaded quicker than uncompressed files.
Once you have downloaded a bundle and extracted the ZIP folder, you will be able to access all PNG images inside the folder. You can then upload one of the PNG images from the bundle when posting on social media.
Here's a visual to give you an idea of what the structure looks like: