If the image is already on the Webplace your cursor over the top...

If the image is already on the Web


place your cursor over the top of it

R/Click in properties open the full URL address and

copy it into the HTML code below

<img src=place image URL in here>

and place it in your post

If the image is not already uploaded on the WEB

Still on your PC etc
Try a free webhost site like PhotoBucket,
http://xs.to/

upload your image and again use the
<img src=??????>

have a practise at it on this thread "Test"