If you click on one of your images you should see a button saying "All Sizes"
When you click that you'll be taken to a page where you can choose different versions of your picture from smallest size to largest:
Large Original (too big to post those here)
Below that is code that you can paste into a comment in discussion threads. Since not everyone has high-speed internet a lot of groups recommend using the small size and then if people want to see a larger version they can click that.
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