There are (2) ways to do that.
When you hit reply or new thread, you will see a couple of icons in the new window.
To the right of the white "SMILEY" face you will see a paper clip. Click on this and you can browse your computer and search for a pic. This will look like the bottle of rum you see.
The other way is to hit the click on the icon that looks like a picture of a mountain. You will then a link to a pic on the web.
For instance "https://cdn-w.eccie.net/images/providers/df/df529a923c18fd38e7d0f38e62e47b a9.jpg" without the double quotes. It will look like the one of you below.
You find a pic and "right click" on it with your mouse and select "copy image location". Paste that link in the window or do it as below..without the double quotes, once again.
"

"