To comment on a specific post, there is a Leave a Reply box under each post. Click on the title of a post to go to it.
Also, or instead, there may be small grey letters at the top left side of the picture. Clicking on Leave a comment there will get you to the Leave a reply area under the picture (and also to a Like icon).
Where these are located depends on what size screen you are viewing on and by which route you got there!
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