You are doing the right thing, but your link may be incorrect. Try uploading to Imgur and opening it in a new tab to get the right link. Sending on Discord will also give you a working link. :)
Click edit on the post and look for the insert/edit image icon. It's the 2nd row and the 4th icon from the right on the bar where all the formatting stuff is. A popup window will come up after you click it. Paste the link there, click "Insert" and it'll do it for you.
Pro tip: Use spoilers if the image is on the larger side so it doesn't swallow most of the page when people scroll through.
Comments on Profile Post by Silken_thread