It's a bit convoluted. Usually, I use the Steam launcher to view my screenshots and a browser to view the forum, so you can easily just copy and paste without closing anything.

1. Go to your game in the Steam launcher.
2. In the right-hand side, there is something called "Manage Screenshots". Click that.
3. Click into the specific image. Find the Upload or View Online button.
4a. IF VIEW ONLINE: click share, then copy the link.
4b. IF UPLOAD: make sure to upload it publicly. Afterward, do step #4a above.
5. Paste the link into your forum post. Steam will automatically render the link as an image upon Post Comment.