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Steemit must pull the first picture from either the summary or the body automatically. I didn't do anything special for this post and the image shows up in the trending timeline. My image was in the body of the post though. Maybe it doesn't work if its in the summary? IDK.

This is correct. It takes the summary and puts it at the top of the page. So if you place an ! + the markdown, it will post it at the top of the page before your text begins.

Oh almost forgot. It seems to work both ways :)