Welcome to the site.
This is how I do it ... Click the More Reply Options ...
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click the "choose file" button ...
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That will take you to your screens and you page through until you have the picture you want. Click on it, then click the "open" button ...
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That will put the photo on the "choose file" tab (1) the click "attach file" (2)
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Then you have the "Add to post" tab ... Put your cursor where you want the photo and click "add to post".
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It shows up on your post like this ...
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But will be a thumbnail in the actual finished post.
You can choose multiple photos by repeating the steps for each one before writing your post. Then you can just put each photo in as you want it.
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When you're ready ... just hit the "Add reply" at the bottom.