Right click the image, then click "copy image url". Then, when writing your forum post, click the "Insert Image" button on the top right of the text box, next to "Insert URL". When you do that, these will appear:
[img][/img]
with the text cursor in between them. Then you can just ctrl+v to paste your image url in between them, like this:
(If you click reply on this comment, you can see how it looks in text form)