To post a screenshot go to Add Reply.
Below the text box you will see "Choose File", click on it and navigate to select the image on your computer.
Click "Choose".
You will then see a file listed in the little window by the "Choose File" button.
Now click "Upload".
To the left of that area you will notice "Attachments" with a pull down called "Manage Current Attachments (1)".
Click on the pulldown and the name of the attachment will come up with a green and a red icon to the left.
Click on the green icon to place the code for the image in your text area.
Note that the image will go where ever the cursor happens to be at that time. You may have to move the code if it ends up in the wrong place.
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This image shows what it looks like with the Golden Wine skin. Some others may have the Attachments buttons in a different place.