I'm going to show you using postimage, because it's the one I use primarily.
https://s22.postimg.cc/6ogojphch/Screenshot_2018-09-06_at_8.04.38_PM.png
Go to postimages.org and this will show up. Make sure it's no expiration and then press 'choose images'
After you choose your image and it is uploaded, this will show up. Copy the link in the second box (the direct link) You might want to click on this picture so you can see it better
https://s22.postimg.cc/idko7rnrl/Screenshot_2018-09-06_at_8.06.35_PM.png
Paste that link into your forum post and voila
https://s22.postimg.cc/i0ta1jfrl/Screenshot_2018-09-05_at_8.13.06_PM.png
You can pretty much put any image that follows the rules on the forums