Welcome to the forum GreyC! These are the steps using a computer/laptop/notebook.
First, the picture will have to be hosted online. some of us use Imgur.com or some other free picture hosting site. Go to the site where the picture is hosted (Imgur, Google etc.) and open the picture. Now, mouse over the URL address, right click and select copy from the drop down box.
Next, come back to this forum and open a response box or start a new thread. Place the cursor where you want the picture to display and click on the "Insert Image" icon from the top of the response box (it's the yellow one).
This will open a box that will ask you to enter the URL of your picture. Backspace off the http:// and right click in the address line. Then select paste and the pictures URL will be in the line. Click "OK" and the picture will show in your message after you submit reply.
It may sound difficult but after a few times it will become very fast.