When you make a post below the screen you type in is a button labeled "Manage attachements" click it.
On the screen that pops up hit "add files"
On the next screen hit "select files"
This brings up you computer. Navigate to the file that is the picture you want and click on it. Then click "open" or similar, I'm working on a MAC. Your computer goes away and then click on "upload files"
You will now be back on the first pop up with your picture(s) displayed in your on forum picture folder. Close that window and you'll see the picture added to your post as an attachement.....(the file name, not the actual picture)
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(picture for illustrational purposes only, I have nothing to do with this rig)