If you have discord, have them send it over discord. Then open it in the browser, and click the insert image thing and use the url from the browser open.
Step 1: they send it to you in discord. Step 2: You click "Open in browser from the discord image". Step 3: You come to the forums and use control P, or the "Instert Image" Icon next to the smilies. Step 4: You go to the browser image that you opened from discord and copy the url at the top, and lastly you paste that into the url thing for the "Insert Image"