In a browser window, start writing your forum post that is to include image.
Now go to
http://www.imgur.com, click "New Post", then "Choose Photo/Video"
and select your image file.
1) it should upload automatically
2) on the next page, hover over the top right of the image, and click on the three dots (...), select "Get share links",
then click the "Copy link" button next to the "BBCode (Forums)" entry
3) go to your forum post, place the cursor where you want the image, and press Ctrl+V